Add 'pixelRatio' property

Using text on Retina displays (or displays with 'devicePixelRatio' > 1) results in blurred text. Also blurred images. I understand that the new 'resolution' property should solve this issue, but I can't get as sharp text/images when set the resolution via PIXI's renderer options. When I scale the canvas manually  (i.e. canvas.style.width = w; canvas.width = w * devicePixelRatio;) I get much sharper text and images, even though the final canvas' HTML element has same dimensions as if using PIXI's resolution. I can't figure the reason why. 

So I introduced property 'pixelRatio' for correct pointers coordinates when the resolution is set to 1, but the canvas is scaled manually outside of PIXI.