Add FieldTracker.diff

[lucasrcezimbra]

Problem

The FieldTracker.changed returns only the previous version. There is no method that returns previous and current versions at the same time.

Solution

Adds a new method diff to the FieldTracker to return the previous and the current versions of the changed fields.

Commandments

• [X] Write PEP8 compliant code.
• [X] Cover it with tests.
• [X] Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• [X] Pay attention to backward compatibility, or if it breaks it, explain why.
• [X] Update documentation (if relevant).