Allow git webhooks to trigger build on git push - Card#287

This is initial part of my work. I've created the main players of this story, meaning:
* updated openshift v3 docs, I've added `/build/schedule` node
* main part of the webhooks responsible for registering, managing and feeding plugins with data

@smarterclayton @bparees please review.
We don't really need any of them, we aren't testing them, and we aren't using them.  So they look meaningful, and they encourage someone to come along and use them.  But if we're not testing them, we have no idea whether they're being used, so they're really just baggage.  Please comment all of them out (except the ones you actually need to read for test cases that correspond to the GitHubWebHook implementation)