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
Enabling collision logging
Revert "Enabling collision logging": silly assumption about dbg()

This reverts commit 01bfde4bc7f969468442c082b9854b5814a4053a.
switched back to debugmsg (disabled)
Merge improved vehicle collision code.