API change for :destroy action + memoization bugfix + CodeClimate badge

- API Change: :destroy action now stores trackers in the format original=value, modified=nil (previously it was the reverse). Agreed with @dblock in PR #70 thread

- 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