Include worker PIDs in supervisor heartbeat/Thrift API

This is aimed at issue #418.

One note is that the new [backtype.storm.util/get-process-pid](https://github.com/strongh/storm/blob/supervisor-heartbeat-worker-pids/src/clj/backtype/storm/util.clj#L319]) assumes that the `Process` returned by `backtype.storm.util/launch-process` (ie from a `ProcessBuilder`) is a `UnixProcess`. This should be true for OS X and Linux, but definitely not for Windows- AFAICT there is no good portable way to get the pid from a `Process`. This isn't an issue as long as running a cluster on Windows is a non-goal.

This is my first brush with storm's internals, please let me know if I missed anything!