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.
In our world, the Mesos master will be responsible for allocating resources between frameworks, and then the Kubernetes master will allocate them to pods within the K8s framework. So, first we'll need to let the Mesos master claim the resources for K8s, then K8s can claim them for a particular pod.

As for updating the K8s-master's notion of capacity on each host, we could update a node's K8s resource capacity once new resources have been claimed for K8s within Mesos, and then write the Binding. This way, the Binding's capacity check should pass, but K8s will otherwise see itself as operating at capacity (until new resources are allocated to it).
(Also thinking out loud)