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.
Again, in a Mesos scheduler, a binding being written doesn't directly drive a change of the location of the pod, although it may indicate that a change of location is taking place or has taken place.