Subscription enhancements

* It is now possible to 'deactivate subscriptions via the delete
  http verb (subscriptions appear to be deleted from the outside)
* Closes #92
* This is not exactly what was specified in #92 but should achieve
  the same result. Instead of introducing a depleted field,
  subscriptions can distinguish if they are fully depleted or just
  temporarily. The method does not involve any overhead.
* Refactorings in the subscription model to increase readability