Rename airbrake to django_airbrake to avoid namespace collision with airbrake-python

Hello!

This renames the airbrake package in here to django_airbrake, since it collides with [airbrake-python](https://github.com/airbrake/airbrake-python). We need both, since we also need a module to report errors from celery which is outside of the regular django request-response cycle, and the name collision was causing problems.

Also updated the README to include the bit about adding the middleware to the list.