Added Mute and Unmute

Pretty simple functionality - unmute only works when a mute has been run immediately before, and works by just using a simple variable to save the original volume value.

For example: 

        spotify.mute(function() {}); // will set the volume to 0
        spotify.unmute(function() {}); // returns audio to where it was
        
        spotify.mute(function() {}); // mutes
        spotify.setVolume(50, function() {});
        spotify.unmute(function() {}); // will have no effect.