Add simplejson to requirements and some tests to help demonstrate the need
Merge callek/build-relengapi:celery-simplejson (PR #143)
Return a non-HTML description for errors
Introduce a 'relengapi' module and 'restapi' service

This service handles display of error messages automatically, avoiding
the situation where a button just does nothing, with the error hidden in
the developer console.
Merge djmitche/build-relengapi:issue135 (PR #135)