Merge pull request #10 from Acanthostega/textures

Textures and widget
Merge pull request #11 from Acanthostega/textures

new cython 0.21
This class is now an abstract class giving a basic interface for buffer object.
We do not need to import Buffer anymore. We need to import vbo and vao instead.
Modified implementation of VBO.
Implementation of VAO.
Adapt HeightMap to the new VBO.
The HeightMap example now use VAO. Still some misuse.
Cleaning and a few corrections.
Remove imports.
Set OpenGL version to 3.3
Modified to use VAO.
Unforced openGL version.
Little correction to textures.
There is now a function to bind AttribPointer.
The build function is now public and should be call explicitly before link. A bind function is now publicly available and should be used to bind everything before link.
Layouts and widget completely working
Adaptation for the use of VAO.
Merge remote-tracking branch 'origin/testing' into testing
Merge branch 'VAO' into VAO_testing