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 http interface doesn't go over etcd, you can talk directly to kubelet. However, it would make some things really tricky, so I'm not sure I'm a big fan. I think I can come up with more alternatives; if you're up for it, a videocall or something might be easier than hashing this out in a PR, though. Some time next week, maybe?