Add support for polling a Kubernetes Master server

This PR adds support for polling a Kubernetes master server and adding new Kubernetes services as SkyDNS A records.  SRV records are also resolved with appropriate port numbers.

This support is rudimentary, and needs some refactoring, especially if we intend to support more backends in the future.
For the Kubernetes team, yes :)  Some of this code is modeled directly from their internal polling routines.