Add tests for field tracker methods in save method

These tests verify that `FieldTracker` methods work as expected inside the `save()` method of a model.

I used a Django signal to inject assertions into a model's `save()` method and I used a context manager to handle the repetitive signal connecting/disconnecting.  Let me know if you think this approach is overkill.

When I cherry-pick 3496fe42916696db36d799facfbf56f9caaa5035 (from #130) onto these changes and run the tests, three of the four tests fail (the .current() function works properly still).