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.
@smarterclayton currently I'm not using any of the fields, but I do see some use cases where we'd check certain fields in the future (eg. author, repository), now it's more of a error check if the request passes the unmarshaling. If you don't mind then I'd leave it the way it is.