added poweroption to stop playback

This will add a new poweroption to stop playback when you press the power button.

There are several user cases where this can be usefull:

1) The RPI is a always on device, but when you press the power button it will shut down and wont' able to sartup anymore untill you replug the power cord. 
It would be more user friendly to stop playback in this case, rpi will be able to blanc the screen then and saves you a bit of energy.

2) If your PVR backend is on the same box, you  don't want to accidently shut down your PC when recording.
Stopping (livetv) playback would also make more tuners available for other clients and/or recordings.

3) Also usefull for other arm (always on) based builds (IOS, Android)

Can someone test this on openelec RPI, I'm only able to build on windows?