Bugfix, fancy viz, better flocking.

visibleNeighbors had a bug making birds blind on one side
if they were heading in a certain direction.
Vector.radialDistance fixes it.

The UI now lets the user pause and unpause, and show the
debug vectors that the birds use to steer themselves.

Birds are all different colors.

boids AI is perhaps working better now, or maybe just
fixing radialDistance is all we accomplished.  Either
way, they kind of hang out in groups that break apart
pretty easily.