Improve Logging in every Delete event.
typo in error.
fix bug. User can delete memberships when they pertain to himself i.e.
when he is the owner.
Additional functionality described in IoTaas paper: Identity Management
in Platforms Offering IoT as a Service.
Update to remove UGLY HTML when wrong HTTP-digest credentials provided
to the API. Now the response is JSON instead of HTML. Thanks to Daniel
Nachtmann! :)