Update README to use aws-sdk gem rather than aws-s3
Merge pull request #27 from kpopper/develop

Update README to use aws-sdk gem rather than aws-s3
adding stub for AR after_commit callback. Fixes an issue introduced by thoughtbot/paperclip@a01677f58762394c2915eb512fe2f69aec6fbcf5 where the use of after_commit was added. This breaks compatibility with Mongoid because they don't implement this transactional callback.
Merge branch 'develop'