resolving issue with now() introduced by django 1.5 as prescribed by: https://github.com/celery/django-celery/issues/222
Updated ApiKey to reference settings.AUTH_USER_MODEL as opposed to django.contrib.auth.models.User
Updated backfill_api_keys.py to utilize django.contrib.auth.get_user_model()
Updated authentication to support Django 1.5's custom user model
Replaced references to BaseUserManager.get(username=username) to BaseUserManager.get_by_natural_key(username)
Updated DigestAuthentication to django 1.5
Additional DigestAuthentication fix
Merge pull request #6 from sbnoemi/django_1_5_clean_for_pull

Django 1.5 compatibility (Custom user models and timezone-aware date issues)