Properly recover from collisions (details below)

We want to bull to get dizzy for a few seconds and be unable to move
except through the force imparted by the colliding object. Then, we want
to get fierce again and resume responding to motion events.

In order to support multiple collisions, I'm using an NSTimer to track
the recovery event--so if we collide again while we're still dizzy, we
can invalidate the impending recovery and schedule a new one.

Obviously this would be much simpler with signals, but I'd like to keep
this project very lean, so NSTimer suffices.