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.
One question this conversation brings up for me is, which system should be the source of truth for resource accounting? And should this source be pluggable in K8S?  Mesos is already (among other things) a generic resource broker.