Check subscriptions

@wesleykendall @jmcriffey 

This pull request includes two functions on the Subscription model-manager

- `Subscription.objects.is_subscribed`
- `Subscription.objects.mediums_subscribed`

Which will return whether or not an entity is subscribed, and all the mediums an entity is subscribed to, respectively.

These methods also handle the case of being given a super-entity, in which case their result will be if any one of their sub-entities is subscribed, or the set of mediums that each one of their sub-entities is subscribed to. There is also supporting documentation for these changes.

@wesleykendall if any of the queries that I'm doing seem to be using django-entity inefficiently, let me know.
are you calling cache_relationships somewhere before this call? otherwise the get_super_entities function will incur another query. You may want to fetch just the super entity IDs yourself using values_list('id', flat=True) (rather than fetching all JSON metadata, etc), and then just do entity_id__in=super_entity_ids.