Django activatable model initial version

@wesokes @jmcriffey @Wilduck

This is the first version of django-activatable-model. It provides the ability to have models that have an active or inactive state and always persist in the database (unless explicitly deleted). It also provides a signal for other apps to know when activatable models have been activated/deactivated.

Going to go ahead and merge this initial version to get it tested in production. The docs should be clear and the code is well tested. Let me know if you have any suggestions and I will update them