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.
Ah, so one last piece of the puzzle: writing the binding causes a state to change in etcd, *not* an RPC to kubelet. Kubelet monitors etcd. kubelet itself does not have to be running at the moment when the binding is written. So the below works fine:

1. T0: Binding is written
 a. k8s master: atomically ensure capacity on the host and write containers to the etcd key /registry/hosts/<hostname>/kubelet, or reject the binding.
2. T1: Kubelet launches/recovers from crash/etc
3. T2: Kubelet watches /registry/hosts/<kubelethostname>/kubelet for containers it should be running

I believe this means you don't need to worry about startup order at all, which sounds like it will greatly simplify what you're doing.