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.
Seems strange that bindings cannot be listed, read, or deleted. Or is the BindingStorage object just supposed to be a log of bindings that were made/attempted/announced? And if it's write-only, then who (if anybody) will actually consume this information?