Django-entity-subscription

Update subscriptions to use the latest version of djano-entity. This includes a switch from relying on `entity_type` to using `EntityKind` references.

Fixed a bug where `filter_not_subscribed` was adding entities that were subscribed, but not passed in as arguments to the resulting list of entities.

A more complete subscription querying API. Now, you can pass in a list of entities, and get back only those who are subscribed to a given medium and source by using the method:

python
subscribed_entities = Subscription.objects.filter_not_subscribed(
source=my_source, medium=my_medium, entities=possibly_subscribed_entities
)

South migrations now correctly depend on `entity` migrations having been run.

Added south migrations.

First public release. A modeling of entity-based subscription management.