BGE: Adding a Python collision API. The initial patch was provided by agoose77, with some edits by me.

KX_GameObject now has a collisionCallbacks list which is a list of callables that are called when a collision occurs. The callables will be called with an argument that contains a reference to the other object involved in the collision (i.e., not self).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60132 954f8c5b-7b00-dc11-b283-0030488c597c