Simplifying the :changeset method on the Version class.
Fixed documentation for custom serializer
Merge pull request #188 from dwbutler/master

Fixed documentation for custom serializer on README.
Adding setter module method for the 'serializer' config option to PaperTrail module.
Removing unecessary 'yaml' require statement from PaperTrail module.
Adding some more thorough test coverage for tracking of destroy events.