Register models both by string name and by type for later lookup.
converter.py now returns model instance for all Create, Update, and Get methods.
Updated ReferenceProperty converter to work well with results from a previous Get/Create call.
Require passing of "WWW-Authenticate" header when raising AuthenticationRequiredError.
Return proper HTTP status codes as appropriate:
  201 - Created (For HTTP PUT response)
  401 - Authentication Required
  403 - Forbidden
  404 - Not Found

Python client updated to handle HTTP status codes and raise corresponding errors.
Updated test code to expect objects back from Create, Update, and Get - previously only expected IDs from Create and Update.