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.
Fix tests to use proper timezone conversions, and remove old-ver restriction on pytz
Merge djmitche/build-relengapi:issue135 (PR #135)
Require Celery >= 3.1.16 to avoid bug
Merge Callek/build-relengapi:issue142 (PR #144)
Merge djmitche/build-relengapi:issue145 (PR #145)
bump version to 1.0.0