Model.Call: Making Assigned_To return a User_Identifier.  (See #146)
Introducing Response.Templats.Forbidden.  (Needed for #146)
Handlers.Call.Hangup: Checking if the user is assigned to the call.  (Close #146)
Merge pull request #153 from sparre/master

Handlers.Call.Hangup: Adjusting implementation to match specification