Add documentation for `preserve_files` option

This option was added to `has_attached_file` some time ago, see https://github.com/thoughtbot/paperclip/commit/65e8d4f6de50732d8e1b.

fixes indentation in code example

Just saw this indentation in passing. Thanks for Paperclip!

AbstractAdapter wasn't delegating `binmode` or `binmode?` to the
underlying @tempfile object. `binmode` is useful for cases where the
Adapter object gets handed on to another library expecting an `IO`
object holding image data. We ran into this with Prawn and people using
`prawnto` in conjunction with Paperclip. Prawn wants to ensure the `IO`
is in binmode for embedding the image data.

Update link in documentation for :s3_permissions.

Use hexdigest to ensure that tempfile names will be a reasonable length.  Remove illegal filename characters check since it is no longer relevant when doing a hexdigest on the filename

When using S3 for storage, their data consistency model makes the
current behaviour of Attachment#reprocess! unreliable.

Apparently, Amazon can not guarantee that PUT's called after DELETE's
is actually executed in that order.

Hence, I propose preserving files, when doing a reprocces.

This ensures that multiline strings are properly matched in their
entirety.

document :s3_credentials being able to take a Proc

Remove references to closed mailing list

http://robots.thoughtbot.com/moving-open-source-project-mailing-lists-to-stack-overflow/

Added changes to instructions to work with Rails 4's strong parameters.