[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.
make all three of these one line:

```
osMux.Handle(whPrefix, http.StripPrefix(whPrefix,
    webhook.NewController(....),
))
```