KAZOO-2560: Camp-on feature

fixed pill request's issue, rebased to master

James, we've thought about your response about sending messages through AMQP and about pivot example. We think we see more generic approach here for calling apps from callflows.
We've added wapi_cf_delegate.erl for this. It resolves dependecies problem for callflow and apps. Please check if it suites well.
Seems like this app will do a lot of work for no reason by listening for all CHANNEL_DESTROY messages. I think a better option might be to use the wh_hooks to receive CHANNEL_DESTROY events per-account, only when an account invokes a camping callflow.

Let me know if you want to know more about that, as I think it might be less taxing on a busy system.