Adding support to pixelRatio (eg. retina display)

Uses body.style.zoom to simulate the effects of pixel density. The
default screen styles were also updated to account for this change by
adding a new property: pixelRatio. Note that old exports still work as
the new property is only applied if it exists.
Merge pull request #6 from filipinascimento/master

Add support to pixel ratio (eg. retina display)