Expose chipmunk joints to python

[PawelRoman]

This is super big topic.

Currently kaa's tree-like node structure creates an illusion that any given node in the structure can interact physically. However it is not possible, a Hitbox must be directly under BodyNode and must be directly under Space node.

In other words, visual representation of the scene in kaa is a tree-like structure while physical representation of the scene must be a flat list of BodyNodes assigned to Space.

This leads to a problem when we want physical structure to be a chain of connected objects.

The solution to this problem is to create a flat list of BodyNodes connected with chipmunk joints. This is more understandable for programmers.

Kaa users will need to understand that kaa's node tree is applicable only for visual purpose, i.e. when nodes somewhere deep down in the tree won't need hitboxes. If a node deep down the tree structure needs a HitboxNode, it means you should not have used a tree at all! You should have built this object as a series of peer BodyNodes connected with joints.