URL functions have moved

Actually they moved in 1.4 but were being re-exported for backward
compatibility. This is removed in 1.6.
Updated to use set_unusable_password

Seems like UNUSABLE_PASSWORD has been removed from __all__ of
auth.models
Added comment on try/except imports
Merge pull request #13 from ox-it/django-1.6

Django 1.6 compatibility