Merge pull request #70 from johnnyshields/master

Trackable#tracked_fields + fixes for Tracker#tracked_changes + Tracker#trackable_parent_class
- API Change: :destroy action now stores trackers in the format original=value, modified=nil (previously it was the reverse)

- Change to Tracker#affected which makes it behave more consistent with the previously released version (I had modified it in my other recent commits)

- Refactor complex method #history_tracker_attributes; create new private method #modified_attributes_for_action