Django 1.6 Support

Fix some of the incompatible changes between Django 1.3 and 1.6
Maintain unit test backwards compatibility

Default.py imports - fix imports to look at new location

moderation/managers MetaClass needs to inherit from type(django.db.models.manager.Manager) instead of type. This is a Django 1.6 change.
See https://stackoverflow.com/questions/21193720/what-happened-in-the-django-1-6-branch-that-affected-how-manager-metaclasses-wor for details

Since Django 1.6 each test is wrapped in a transaction, so if an exception occurs, the transaction is broken and you need to rollback manually. The solution is to wrap the exception code with transaction.atomic(). In those test cases, check that transaction has the 'atomic' attr - if it does use it.

CustomUser extends User instead of AbstractUser in the test models.

Update ModerationAdmin.get_form so that the function prototype matches the super. It was causing problems with 'fields' kwarg missing

Test configs:
Add Django 1.6.5 to travis
Update Django 1.3 to 1.3.7
Add django1.6 targets to tox