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.
The method I'm proposing here is that bindings will be write-only. They needn't be stored anywhere--the Host field of the binding will be reflected in the host of the pod once the binding is accepted.

The BindingStorage object here is just going to implement the application logic. It's not actually going to store the binding. Admittedly this is confusing, but the *Storage suffix is here for consistency to indicate that this implement's our apiserver's RESTStorage interface.