add support for multichannel audio output

[billorcutt]

add support for multichannel audio output

[billorcutt]

http://momdev.se/interactivemusic/test/multichannel/myscript.js

[billorcutt]

This is done. Works like this (send four different oscillators to four different channels)
__().sine().multi_out(0);
__().saw().multi_out(1);
__().triangle().multi_out(2);
__().square().multi_out(3).play();