fabric.Object.fillRule support using globalCompositeOperation

Full support for the fabric.Object.fillRule option for any visual objects.
_setupFillRule(ctx) is used to set CanvasRenderingContext2D.globalCompositeOperation (from fillRule property).
_restorFillRule(ctx) restores previously saved globalCompositeOperation.
Both methods are called form the render() method, so individual blending settings for each object can be specified.
Merge pull request #1172 from kreig/patch-1

fabric.Object.fillRule support using globalCompositeOperation