Adding tests and logic to ensure that tempfiles are closed and unlinked after queued_for_writes finishes uploading/saving

Decouple the Interpolations class from the Attachment classs, you can extend Paperclip with custom interpolations as paperclip-extended does.

Abstract out the file geometry parser within Thumbnail. This makes it possible to have different cropping and scaling controls, as the papermill gem does.

This will prevent someone to think that Paperclip test suite for master branch is currently failing, as the build was failed on a WIP branch, not the main branch.

http://travis-ci.org/#!/thoughtbot/paperclip/builds.

I think it's OK to have the bugfix for MongoDB in the plugin.

This reverts commit cde22564.

Add rubinius to travis testing

Bump rails 3_1 version to rc6

This reverts commit fe01ae69.

It seems like that JRuby on travis-ci is broken[1], so we shouldn't test against it to reduce the false-positive result. I've run the test locally, and it passed just fine[2].

[1]: http://travis-ci.org/#!/thoughtbot/paperclip/builds/79181
[2]: https://gist.github.com/299686b412f9ad878e16

Update Gemfile to make sure we can run the test locally, and also upgrade Rails 3.1 beta to latest version

Closes #534, closes #538