Fix bug 1082031 - Updated Celery to 3.x including its dependencies.

This also updates a bunch of dependencies:

- billiard to 3.3.0.19
- celery to 3.1.17
- django-celery to 3.1.16
- kombu to 3.0.24
- librabbitmq to 1.6.1

We also remove the previously overridden South migrations for django-celery and start to store task results in the database instead of RabbitMQ to prepare for the eventual move to Redis as the broker and the ability to see task results online.