Fix instance.save(update fields=[...])

This adds a test for using the new `update_fields` in the model `save` method in Django 1.5 and modifies `ModelTracker` to pass the test.

This isn't ready to merge yet because there is still no test for `update_fields` when saving the model for the first time and I'm not yet sure how Django models handle that case yet.

Fixes gh-28.