Add line in support

Sam,

This adds basic line-in support in the format "@playing_speaker.line_in(@source_speaker)".  Note, this only changes the input and does not play sound.  To play sound, this needs to be followed by @playing_speaker.play since the default is for the input to be stopped.

I haven't yet figured out how Sonos filters the list of speakers to the ones with something connected to the line_in input. 

Regards,