Change to not render blocky text on retina displays on iOS

CATextLayer is [known for rendering blocky text on retina displays](http://stackoverflow.com/questions/5538394/vector-like-drawing-for-zoomable-uiscrollview). The fix is to set its contentsScale to match the screen's scale.

Note that we can't easily do the same for retina on OSX because the layer [may be rendering on a screen with different resolution than the main screen](http://blog.mazaheri.me/post/65931558994/cocoa-please-never-draw-your-nsview-nscell-and).