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.
I like the idea. I think this should be named differently, as I think the idea is to pass control of a call from one arbitrary app to another (could be the same app but a different instance).

Let's call it wapi_delegate.erl and put it in core/whistle-1.0.0/src/api since we can't rely on the callflow beams existing for other apps to use.