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.
Since the Mesos master may need to do its resource accounting before the Kubelet launches the pod (to prevent other frameworks from claiming the same resources), and the Kubelet may not even be running yet, I don't think the onBind mechanism would work for us.

Is the http interface you mention an abstraction over etcd, or a kubelet-only (etcd-free) interface for launching a pod?
Even if it's just an abstraction over etcd, we would greatly appreciate such a non-etcd-specific interface to the shared state, so we could potentially make use of our existing Zookeeper or MesosReplicatedLog storage.