[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.
Could use a better name...  GetBuild?  GetBuildFromPlugin?  GetBuildFromWebHook?  (referring to the "webhook" is confusing because it's the webhook that called us, we're not calling the webhook).  GetBuildFromWebHookPlugin is probably the most accurate name, but probably too long to be reasonable.