[Feature Request] Add a handler to trigger upon device change.

[dsancheznet]

Hello,
maybe it would be possible to add an event driven handler to be able to trigger changes in device states. Currently I'm working on a GUI based on sonoff-python and I have to trigger an update manually or timer-based.

Also it would be nice, if the instantiation wouldn't block the execution - although I can resolve that with threads or subprocesses...

Overall, this library s a nice job, guys!

Best Regards
D.Sánchez