Wait, I need to also include the sender on the notification, because
DUH, of course I do.
Added a clock class, which uses threads. This supports tempo
changes, which gives us a new variable to play with. Tempo changes
lag behind the beat- they'll happen after the next beat happens.
That's probably desireable behavior.
Documented the clock class and changed its interface to support a
ticks_per_second property, which is clearer than set_speed.
Simple listening composer, which can pick up events from other
composers.