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.
I was actually thinking that it would call AssignPod directly; consumers of this information would get it by watching pods--their Host field will change, and the reason that they're waiting would change. More layers of indirection can be added, but it's hard to remove them later, so I'd rather start simpler.