Many updates

* Nicer Earth texture
* Implement clouds
* Use bilinear filtering instead of linear
* Port model loading to Cython
* Disable context shadowing
Optimized some code.
Merge branch 'master' of https://github.com/Icyene/SpaceTorus

Conflicts:
	setup.py
	space_torus/_cyopengl.pxi
	space_torus/game.py
	space_torus/glgeom.py
	space_torus/world.py