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 missing piece here is that there may not be a Kubelet already running on a Mesos slave initially, so I think we'll first go through a message cycle where KubernetesMesosScheduler calls MesosMaster.launchTask (to allocate the resources to Kubernetes) which calls MesosSlave.runTask (to download and launch the Kubelet + Executor, if necessary). Then either the KubernetesMesosExecutor or KubernetesMesosScheduler can write the Binding, triggering the (now running) Kubelet to observe the Binding and start the pod.
Thanks for the clarification. I think we can work with this model for now, but I can imagine a future where Mesos would want to launch pods itself. We can cross that bridge if/when we get to it.