Collect some stats

This PR will set Panel up to collect various stats about breaker activity.

- [x] Publish trip/reset counts to a Statter interface
- [x] Replace `OnTrip` and `OnReset` callbacks with a channel based event stream
- [x] Make the stream a little more robust in the case of absent/blocking listeners
- [x] Possibly replace Panel's callbacks with event streams?
- [x] Update docs and examples

See also #13