Add support for Django 1.5 - All unit tests pass for Django 1.4 and 1.5.

This fixes #1609, #1607, #1608 and #1650.

- Support for Django 1.3 dropped.
- Support for Python 2.5 dropped. (The code currently still works with Python 2.5 and Django 1.4. Since Django 1.5 requires Python 2.6 or better the official support for Python 2.5 was dropped and the CI configuration no longer runs the tests with Python 2.5)

Please review my changes to the reversion code thoroughly. I have never user django-reversion before and had a hard time figuring out how to convert the old API calls to the new ones.