Further simplification of the WebGL renderer.
* Sprite batching working
* Shaders organised into a folder with a _BaseShader base class.
* Finished removing static globals by switching the GL graphics handler to instance based.
This should make it easier to have separate render contexts later and to deal with context lost/restored events.
* Untied renderer specific data from the DisplayObject derivatives, the primary one was
Graphics which had some GLData associated with it, now it is stored in a map inside of _GLGraphics instead.

Added a port of the bunny benchmark (as adapted by Matt DesLauriers to gauge batching performance with multiple textures) :
https://github.com/mattdesl/pixi.js/tree/culling/examples/bunnybench