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!

Remove the Options class, replacing it with a hash.

This will fix the problem on the CI server, where some time the change occurred by "write_file" will not trigger Rubinius to generate a compiled .rbc file.

This also removes Pry.

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

Fix 293, 601: STI subclassing overwrites options