Webhook backend

[jonbackhaus]

Proposed webhook backend; tested against Slack

[coveralls]

Coverage increased (+0.2%) to 81.366% when pulling 6d2ba5e on jonbackhaus:webhook into ecfeee9 on dschep:master.

[dschep]

Not a bad idea. However, I think the payload should be JSON encoded. eg payload = json.dumps({"title": title, "message": message})