use internal tick callback to report collisions

Currently it's possible that not every collision event is reported if Physijs.Scene.simulate executes multiple internal simulation sub steps.

The correct way to check collisions is during a internal simulation tick callback:
http://www.bulletphysics.org/mediawiki-1.5.8/index.php?title=Collision_Callbacks_and_Triggers
http://www.bulletphysics.org/mediawiki-1.5.8/index.php/Simulation_Tick_Callbacks

I changed only collision reports to be executed during a sub step because it was obviously misbehaving. Other report types could also be changed but that's not necessarily the desired functionality.