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.
From your description, it sounds like what I've phrased here as "writing a binding" is analogous to Mesos' "call runTask/launchTask on master".

If we're trying to run a k8s master and a Mesos master at the same time, I can think of a couple ways in which that might be possible:

1. k8s master defines an onBind hook, which Mesos master subscribes to.
2. Mesos scheduler starts tasks via Mesos master, which pushes containers to kubelet via its http interface instead of via its etcd interface.

I'm not sure if we'd really want to fragment the mechanisms as in 2, but it's possible.