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.
That would probably work, even if it's a bit rube-goldberg-like. I'm concerned that k8s master may reject the pod to begin with due to thinking it's at capacity, though.