Added cacheAsBitmap to PIXI.Graphics

Tweaked webGL graphics
Added generateTexture function to PIXI.Graphics
Merge branch 'DisplayObject-Cache' into dev

Conflicts:
	bin/pixi.js
tiny tweaks - removed replaced [] with length=0 in PIXI.destroyTexture
Merge branch 'dev' of https://github.com/GoodBoyDigital/pixi.js into dev
Merge branch 'dev' of https://github.com/GoodBoyDigital/pixi.js into dev