Add itunes shuffle command to OSX plugin.

Added shuffle functionality to the itunes command in the OSX plugin.
iTunes 12 doesn't support setting the "shuffle" property on the playlist
from what I've read, so this simulates the user interacting with the
iTunes interface. The toggle option "clicks" the shuffle button in the
Now Playing area. This has the effect of toggling the shuffle setting on
whatever is currently playing. The on and off options use the Shuffle
menu bar items.

Usage: itunes shuffle [on|off|toggle]