WIP Card#287 - Allow git webhooks to trigger build on git push

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.
Let me lay out what I'm thinking here.  First, what is the point of a build web hook?  It's to take an incoming request and either validate it, or return an error.  If there's data in the the web hook that is relevant to the builder, that needs to be extracted.  So that sounds to me like a web hook is a plugin (interface) that takes a request and returns either a build object or an error:

    Extract(req *http.Request) (api.Build, error)

Secondly, there's an object that is responsible for figuring out which web hook to invoke.  This is an object that wraps a map of plugins by name and returns an error if the plugin isn't defined.  So that's an http handler that expects to get a request of the form `/<plugin>/*` and passes everything down to the handler.  You don't want the handler to have to know about the plugin prefix, since it's possible your web hook might have additional data.  This handler should probably just return the build object the web hook handled or the error if the plugin isn't defined instead of writing a response (I don't think web hooks are responsible for writing their own responses today).

Third, you need to have a top level object that gets information about a buildConfig by its id and checks that the security token passed in the URL matches the build config.  For now, you can make the token be unchecked, but it needs to be part of what happens.  The first two objects shouldn't know about this token.  This should also be a handler, but it's much closer to a top level REST resource.  This is the object that needs to handle the build and error that the web hook / handlers define.  It should then create the build.  The two other objects shouldn't know how to actually run a build, they should just be responsible for creating a build object.