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.
I guess I would view writing a binding as the act of attempting to claim resources. If this needs to result in a claim in the Mesos system, then we need to add a hook or something. Having two such claiming steps seems racy to me. (I suppose another alternative is to allocate/claim the resources in Mesos, and then write the binding to k8s as a formality to get the master to assign the pod to the kubelet. You could report infinite/very large resources on the kubelet to make k8s master's check always pass. However, this would potentially make k8s' hypothetical admission logic not know what it has resources to run.)

> At T0, if no Kubelet is running on that host, how will the k8s master "ensure capacity" on the host?

We'll need to have a previously stored record of the host's capacity. Then we can add up the containers assigned to it and compare. We haven't developed a capacity reporting system at all yet. It sounds like to make this work, we'd need Mesos to update this in response to other frameworks claiming the resources. I'm really not sure how that would work within Mesos.

(Keep in mind I'm thinking out loud here.)