Proposal for scheduler API

This commit adds a Binding object. The idea is that schedulers can write
these to cause pods to be asssigned to hosts. No implementation is
provided; if this is accepted, I'll write that once we get pods moved to
a different etcd key.

This continues k8s' tradition of phrasing all APIs as RESTful handlers.
So, is the idea that you write a Binding (to etcd) and then etcd/whatever passes it on to any Watcher on the onBind hook? That's what I was trying to get at when asking about the consumer. I'm now understanding that once written, the Binding is not directly accessible through the etcd/shared-state Get/List/Delete interface, but is internally passed on to any Watchers who will consume that data as they like, with AssignPod in particular updating the 'host' field in the actual Pod entry.