incorporate k8s scheduling backoff logic

implement fix for #50 
As part of this PR I've simplified the scheduler: scheduling is now completely driven by the kubernetes scheduler plugin API, as opposed to being driven by both that API as well as mesos. This makes scheduling much easier to reason about. It does not, however, change the greediness of the scheduler.

That said, we should be less greedy. I was thinking about tagging offers with a TTL, after which we'd reject the offers. We could even allow this TTL to be configured via some command line flag (this sounds like it merits its own PR). Thoughts?