[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.
My comment with structure would instead be:

```
controller := webhook.NewController(map[string]webhook.Plugin{"github": github.NewWebHook})
osMux.Handle("/buildConfigHook", http.StripPrefix("/buildConfigHook", controller))
```