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.
Yeah, PUT/POST to /bindings.

The binding itself has no ID. It's currently just a (podID, Hostname) pair. It could expand later to list the resources that are being bound.

Maybe we'd eventually have to track bindings as separate objects, but for the moment I was just planning on finding the pod and updating the hostname (and moving its containers to the right kubelet, etc).

This is intended to be the "scheduling actuator" in the architecture diagram.