Adding a new delay() method to the RTC class for milli-second delay.
Cleanup.
Improving handling of NEXA::Receiver listeners.

1. Introducing a Listener template class.
2. Refactoring NEXA::Receiver::on_event() and dispatch() to use Listener.
3. Updating example sketch CosaNEXAreceiver with the latest.
Reduce amount of documentation per class. Removing private sections.