Replace custom subscription with entity-subscription

@wesleykendall This pull request removes the subscription logic that was internal to the entity_emailer app in favor of entity_subscription.

While this does require some additional work for the consumer of this library, it allows for more fine grained control of subscriptions.

A number of other housekeeping changes are made in this pull request as well. Including:

- adding a optional `from_address` field to the email model (which overrides the default from_email).
- adding south migrations.
- adding the possibility for configuring the emailer medium, and a management command to create it if it doesn't exist.
- updating the documentation.