Trade Django 1.4 compatibility for Django 1.6 and Python 3 compatibility

- Tests pass under Django 1.5 and 1.6, Python 2.6, 2.7, 3.3, and 3.4.
- Tests fail under Django 1.4 because `six` was not included in Django until 1.5
    - I could add `six` as a separate dependency instead and maintain compatibility with Django 1.4 if that is preferable