start/stop object tracking from command line

/to @rmg or @kraman 

I need to update README, and read my own code over again, and do some manual testing on usability of cli messages, but it appears to  work.

Also, in tests, I'm not verifying that the metrics are sent... should be easy by listening on the UDP metrics port, but that's for tomorrow.

But basically, its here, ready for pre-review.
This is starting to look like it could use extraction to a mapping of commands to functions..