The original commit came from @amazonwebservices and @trevorrowe in #579.
I had to revise the commit and make sure all the test cases are passing.
All credits still goes to those guys, thanks a lot!

Separate the UrlGenerator out from the Attachment. Some example plugins that could be written include generating thumbnails on the fly for different thumbnail sizes, or delaying the thumbnail generation until it is first called.

Fix a bug when passing a method name to `:if` and `:unless` option in `validates_attachment_presence`

Fixes #631

ones (:private and :public_read) in the s3_test
completed test for lambda permission value

bit of work

Closes #602

So instead of having:

    http://s3.amazonaws.com/paperclip/var/www/foo.com/current/avatars/1/original/foo.jpg

because default path has the :rails_root, now you'd have:

    http://s3.amazonaws.com/paperclip/avatars/1/original/foo.jpg

This shouldn't affect your application because you'd normally set the path to some string.

Paperclip::InterpolatedString will ensure that we're not double escape the string, which was the cause of the escaped filename problem in #path method.

Added comments and add explicit ordering to this method, which makes the result deterministic and resumeable should an error occur mid-way. 

As values in set are strings - check is unnecessary.