**Warning: Critical change on Physics Collisions

A) Now for use when_collide method of the World instance, you need to
pass an instance of a PhysicsSprite and it will return in the block the
colliding physics sprite if available, or the body that is colliding.

B) If you still need to get the collisions of the physics bodies you can
use the method on_collision of the World instance, it will return the
bodies colliding and if they are touching