Implement a notification system

@HydrelioxGitHub and @rytilahti proposed that we implement a notification system that will allow the library consumer to listen to notification messages from the bulb and update its state in realtime. It's a good idea, and I would like to see this implemented, but there are various caveats and ways this could be implemented, both internally and in the library API.

This issue is for discussion on how we could do this.