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.
We are probably going to break the cf_endpoint code out to a core lib, since more apps are using it to build endpoints like this. Just be aware, and we'll try to make sure to announce the move when it happens.