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.
This feels weird from a Mesos scheduler perspective, since when a Mesos scheduler decides which machine to run its pod on, it will tell the Mesos master to launchTask, which will then tell the slave to download the kubelet/executor and call runTask. We wouldn't want to do the binding/pod-assignment through etcd itself, since we need the Mesos master to be able to do resource accounting for the task/pod being launched.
Or maybe I'm misunderstanding what it means to "cause a pod to be bound to a host." If this just means announcing which machine a pod will be/is launched on, then a Mesos scheduler could announce its binding either once the scheduling decision is made (Assigned) or after the kubelet is ready to run the task (Task_Starting in Mesos TaskStates). But we would not want the Binding object itself to be expected to be the driver of actually launching a pod.