Support shutting down a cluster

- Add 'stop' command to cli
- Add 'finish' API to controller

See question in comments on #13, how to shutdown a cluster.

SLN-562

/to @dirkmc, what do you  think?

I'd prefer to make `control.stop()` shutdown all the workers, if-and-only-if setSize had previously been called (indication the controller should be in charge of spinning workers up and down). But, that's an API change. Maybe I can introduce it in the next minor release, but not in a patch.