Added infrastructure and test for return_type decorator
Simplifying approach for return_type decorator.

No longer requires changes to the server infrastructure - just automatically
runs the requested function on the output from the service function, and returns
an error if something went wrong.
Adding a test for a bad converter function
Merge branch 'return-type-decorator'

Closes #293
Converting runtime error-signal to HTTP error for return_type
Using tangelo error codes instead of cherrypy
Augment calculator example (and docs) to check for ajax error
Merge branch 'decorator-server-error'