Change UIImages to be created at retina scale

As of iOS7, all devices sold are retina, so it makes sense to transition to loading all the images with the proper scale.  This makes it easier when using UIImageViews to use the more interesting view modes, like top-left, bottom-right, etc.

If you merge this directly, you'll break people who depended on the old behavior though.  You might want to make it configurable with the default being 2.0.