Setup Turntable.fm style mute functionality via space bar.

Hey, just came over from TT.fm and love your plug-in. Wanted the ability to simply press the spacebar anywhere on the body of the page and have it mute the music. 

I'm not sure how you like to organize your code but I just dropped in a "setupMute()" method and then called that in the PlugPlusApp constructor instead of putting it all in the constructor. I also put this in a "dev" branch since I figured you'd want it there. Let me know if you like this or if you'd want to change how this is setup!