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

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.
Not familiar w/ the apiserver split path method @smarterclayton is referring to, but i'd second that this looks like an over-engineered approach to handling the REST path.