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.
The plugin should either return a new build object (best) or handle creating the build itself (if, for instance, a certain type of build hook works best if it can invoke some other mechanism).