Handlers.Call.Originate: Bug fix and clean-up.

Fixing bug in parsing of reception contact identifiers.
Reorganised Generate_Response.
Substituted protocol description with a reference to the protocol specification.
Terminology update in package Model.
Image and Value functions for type Model.Reception_Contact_Identifier.
Using Reception_Contact_Identifier instead of separate Contact_Identifier and Reception_Identifier.

AdaHeads / call-flow-control