[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.
Your controller should have a reference to a "build.Registry" (as defined by cesar's code) and you should call it directly here.  Once his pull is merged you can rebase on top of this.