[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.
In general yeah, you only get to define an endpoint that is posted to.  Occasionally they'll take basic credentials for the post.  We can start here, and in the future allow plugins to check the secret themselves.