Documentation update for thoughtbot/paperclip#2217.

[These lines](https://github.com/thoughtbot/paperclip/blob/95acf3b898bd782f4429d42fd2d8812505390328/lib/paperclip/storage/s3.rb#L101-L112) in the documentation indicated that the value for the `s3_storage_class` configuration option should be `:reduced_redundancy`. However, using this value with Paperclip 5.0.0 results in a `Aws::S3::Errors::InvalidStorageClass` error with the message `The storage class you specified is not valid`.

The [documentation for the `Aws::S3::Object#storage_class` method](http://docs.aws.amazon.com/sdkforruby/api/Aws/S3/Object.html#storage_class-instance_method) in the `aws-sdk` gem documentation indicates that the correct value is now `REDUCED_REDUNDANCY` (along with a few other potential values).

There's a subtle difference between what `\Z` and `\z` consider the "end
of string" which is that the uppercase version allows a single trailing
newline:

```
/\Afoo\Z/.match("foo\n")

/\Afoo\Z/.match("foo\n\n")

/\Afoo\z/.match("foo\n")
```

* There is no need to pass options argument to UrlGenerator
* Houndci compliance

I recently implemented a custom processor that used the Exifr gem to
extract EXIF information from images uploaded to a Paperclip attachment.
Exifr's processor uses readbyte to parse the EXIF header, so it hit
errors when Paperclip's File-like object didn't have one. There's also a
test for this delegation.

I've also tidied up the README documentation for custom processors,
hopefully to be more clear than before. There was some duplicated
content between the "Post Processing" section and the "Custom Attachment
Processing" section, and those sections were separated in the file. I've
dedicated the "Post Processing" section to Paperclip's built in
thumbnailing processors, and made "Custom Attachment Processing" section
solely about writing & using your own custom processors. This should be
a more understandable progression, as built-in functionality is
discussed first & separately from extending Paperclip.k

- The uri io adapter now seeks for the content-disposition header
  if this is pressent the value filename is taken instead of the
  last path segment for the resource file name
- Fix style comments
- Applied the Tute Costa refactor to URI Adapter.
- Added entry to  the NEWS file.
- Removed editor tracking file
- Fix test cases

Before this change, the value for `preserve_files` was the string `"true"`, potentially confusing users that they should use stringified booleans like `"true"` and `"false"`.

However `"false"` would evaluate as truthy, and preserve files.
This appears to have been a factor in https://github.com/thoughtbot/paperclip/issues/1445#issuecomment-51430186

AWS no longer accepts `:aes256` as an encryption value and will raise
`Aws::S3::Errors::InvalidArgument` with the message `The encryption method
specified is not supported`. It should instead be `"AES256"`.

This commit makes no implementation change, but for those that use
source code and tests as documentation, it should help prevent
confusion.

Release v5.0.0

Update NEWS file.

[ci skip]

* Add `read_timeout` option to `UriAdapter#download_content` method
* Default `read_timeout` is `nil`
* Update NEWS

* replaced if-not-blank with if-present

an instance of code was testing presence via `!a.blank?`. this is now
replaced with a more readable version: `a.present?`

* replaced 2 if-not-blank checks with if-present checks

* temporarily restrict travis

* reverted travis changes

* readded whitespace

Add test to prevent regression regarding excessive logging

Fix a nil error in failure_message of content type validation matcher

[ci skip]

When a matcher uses both `allowing` and `rejecting`, and validation fails,
`@missing_rejected_types` might still be `nil`.

When a matcher only uses either of the two, a part of the message is `nil`.

Note: this is most likely not the way we want to test our matchers, but it
demonstrates the current issue.

Excessive logging can fill up disk space and become a denial of service attack, see https://cwe.mitre.org/data/definitions/779.html

[fixes #2016]

This is part of the effort started at
https://github.com/thoughtbot/paperclip/issues/2199 to ensure the test
suite is green locally.

The attachment processing spec was failing since the setup required done
in a before block, namely a call to `rebuild_class`, was running
restricted to one of two contexts in the test file.
This was leaving the `Dummy` class in a bad state and affected other tests.

This PR moves the `before` block to run for the both of the contexts.

Correct validators spec

This is part of the effort started at
https://github.com/thoughtbot/paperclip/issues/2199 to ensure the test
suite is green locally.

The validators spec was failing since the setup required was missing a
call to 'rebuild_class' and leaving the Dummy class in a bad state.

This PR adds the call to the before block setup.

This is part of the effort started at https://github.com/thoughtbot/paperclip/issues/2199)
to ensure the test suite is green locally.

This spec was failing on obtaining a S3 instance with an `nil` region
key. Since `Aws::Resource` does a regex match against the region key
passed in, we would fail on `NoMethodError`.

This corrects the issue by setting a region key to an empty string in
the test setup.

[fixes #1932]

Capistrano won't symlink the `public/system` folder by default anymore.
Capistrano needs to be configured accordingly.

See related issues:

* https://github.com/capistrano/capistrano/commit/033bf8ee492936787cd0744e69b3df8a5ad5b6e6#diff-348edd3e734eaeed0fe97af037f3c647
* https://github.com/capistrano/rails/issues/104
* https://github.com/capistrano/rails#symlinks

[fixes #2141]
[fixes #2166]

Clarify semantics of the only_process option (documentation-only)

This was really confusing to me, and I think this small change would help to clarify what's going on without having to trace through the code.

README elucidation for Ruby beginners

change 'show view' to 'view helpers'

Clarifying contributions.

Update README.md

Fixes #2130.

Added FactoryGirl configuration details