Use webgl rendering context

[GMartigny]

Supporting the WebGL rendering context would greatly improve the overall performance of Pencil.js rendering loop.
WebGL also open a lot of new features like 3D or complex shaders.

This would be a huge modification to the code-base, because everything assume the 2D rendering context is used.
Since WebGL is supported in virtually all browsers, it could be the default renderer.

Adding this should not impact syntaxe as much as possible.