[WIP] 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.
You can pass your NewController directly to httptest.NewServer() since it implements http.Handler.  Also, you probably want to pass plugins in to it.  Wit those changes this becomes a 1 line function and you should just inline it:

server := httptest.NewServer(NewController(...))