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.
Actually - I take that back.  You want your plugins to implement a method that takes a request and returns an error (as you defined above).  The build webhook handler needs a method that takes a request and response and a build id as arguments.  The person using your build webhooks is responsible for parsing the request to get the build id, and then invokes that method.