[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.
This is probably all a bit too complicated - you should just reduce this to a map[string]Plugin and invoke the plugin directly.  Instead of calling getBuildFromPlugin, just look up the plugin in the map and invoke a method.  At that point you don't need this file.  PluginFactory() is also unnecessary.