Fixed three issues blocking 3.3/1.7 support

- lookup loading in 1.7 uses the new autodiscover helper
- auto-complete fields ensure unsaved identical objects are not
  considered changes (1.7 no longer compares them equaly)
- defining __str__ for 3.3 on test Models so that escaping works
  properly
Merge pull request #126 from ironfroggy/feature/support-django-1.7b4

Fixed three issues blocking 3.3/1.7 support