Vehicle collisions no longer become insane ping-pong fests that generate velocity out of nowhere

Vector changes now derived from equations at http://en.wikipedia.org/wiki/Elastic_collision

Please test, it may need adjustment.

As is, it may be slightly too far in the opposite direction, but don't expect to budge a semi with your motorcycle, nor fear it rocketing back after reaching mach 20+ due to recursive multi-vehicle collision sprees.