Enhancements:
- Check whether play/pause/stop/next/previous are currently available before issuing command to ZonePlayer to prevent UPnP 500 errors;
- More consistent log messages ("changed" and "set" from HomeKit perspective, as for other plugins);
- Leverage homebridge-lib for custom services and characteristics;
- Check for latest published version in NPM registry;
- Only call copyCoordinator once per Sonos event.