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.
Hehe, it's on my list to rename the files in this directory.

e.g., pod_registry.go should actually be podstorage.go.

I think it was @proppy that was making the argument that _ shouldn't be in the names of go files. Browsing through the standard library a bit, it looks like they're exclusively used for *_test.go and *_decl.go files.