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.
We aren't going to use global registration functions - instead, you need to make the webhook handler be a NewWebhook(plugins) http.Handler

People who want to use you will then build the map themselves and then register your handler.