add remote poweroff feature

Hi,
I want this feature for a long time, it can help me save the energy, and let my room cool down, even save money, I think it's a really good feature, hope you can merge this commit into this project, if you will accept with PR, **be careful because it depends on poweroff feature which was not provide by original version, and I've add poweroff feature in #13, **you can merge #13 before this commit(if you agree with that #13), or add poweroff feature by another commit before this((if you don't agree with #13), by the way, I am working on another feature - poweroff after a successfully oneunit folding, it can save more energy and money for F@Hers :)
Thanks!
Peter