Add ModelTracker with tests
Move ModelTracker to tracker module
Remove unnecessary use of setattr
Rename current_fields to current
Merge ModelTracker and descriptor classes
Fix return value of ModelTracker.current pre-save
Add tests for ModelTracker.current method
Add tests for multiple model trackers
Add ModelTracker note to CHANGES
Add Trey Hunner to authors file
Make ``from model_utils import ModelTracker`` work
Add ModelTracker to README file
Add test for ModelTracker descriptor on class
Add blank line to end of README to avoid reST error.
Specifically acknowledge Trey for ModelTracker in changelog.