Compatibility with the custom user model

The south migrations freeze the user model, because of which the custom user models (as introduced in Django 1.5) raise errors on creating a new job.

As a workaround, I froze the table names inside the model - this ensures compatibility with existing deployments and also makes the maintenance easier.

For south migrations, an hacky workaround was needed as suggested here: http://kevindias.com/writing/django-custom-user-models-south-and-reusable-apps/

I have incorporated the ideas from the above post in this commit.

Do let me know if more details or documentation is needed.