Immediate response

Depends on #19: If we succeed with making Bender somewhat useful, we'll probably continue to use a lot of integrations and REST calls. Naturally, some of those calls will take a pretty long time - Bender should probably respond right away, letting the user know that he's dealing with the request, to deal with that 'is Bender even there' feeling. That's easier said than done and does require outgoing webhooks, enabling Bender to post to Slack without being prompted.