s3 url doc improvements

This should fix the problem on animated file detection.

Fixes #851

This should fix an error when the user has turn on AWS logging while
and upload a file with non-English filename, as `aws-sdk` logger is not
encoding aware.

The problem was that the string returned from another logger methods
is in `US-ASCII`, while the string returned from Paperclip would be in
`UTF-8`. When the logger calls `#join` on those strings, Ruby would
raise an exception.

This solution is not the best, as it involves a monkey-patching, but
should make sure that the upload goes through without any hiccup. For
the long run, I'm going to send a patch to `aws-sdk` and also make
Paperclip require at least `aws-sdk > 1.4.1`

We've changed some of the setting and some of the S3 tests got ignored.
This should make sure that the tests are really passing, after the
changes to IO Adapter.

This will prevent a pitfall as sometime the style's name can be a string.

Fixes #762

Fixes #846

Fixes #843

`aws-sdk` has been updated to support newer version of nokogiri, so we
should loosen our dependency.

* Force column reset in Rails >= 3.1
* Reuse another helper method

ContentTypeValidator now honors the `:allow_nil` and `:allow_blank` option.