Free rotation code and example

[miq75]

Hello,
I need to draw origami folding methods in strict python, so I use your basecode and add some little more features.

My current changes

• Minor corrections in cubeExample faces and animation;
• Minor optimisation in Engine.render;
• Adds in graphics/vertex.py:
  Vertex.free_rotate() based on quaternions methods to avoid numpy use.
• Adds in graphics/engine.py:
  Engine.free_rotate() and Engine.part_free_rotate()
  Engine.__next_fold(), Engine.__prev_fold() and Engine.__fold_animate()
  Fold animation consts definition and N and P key bindings in Engine.init()
• Add of foldExample.py

Take it if you think they're cool ;)

By the way, do you think that

• more than 3 vertices polygons can be used (implies many changes of variable names and faces vertices management) ?
• there is a way to have a better 3D render method that perfectly manages almost paralleles faces (avgZ method sometime fails, showing small faces above bigger ones) ?

Thanks.