Fix for #57 (UIImage vs. NSImage for OS X)

This fixes #57 by using ifdef's to work around the fact that `UIImage` and `UIScreen` aren't available on OS X. What it doesn't do is account for the considerable complexities involved in properly handling Retina images on OS X. For now it just does 1x all the way. There might be some subtle mis-behaviors associated with that (maybe filenames in cache?), but at least it compiles, runs, and looks pretty reasonable on OS X.

Also worth noting: Getting this to compile and run on a Mac requires a provisioning profile which includes a device UUID for the Mac you're trying to develop on. It's like running code on a developer's iOS device, or doing ad-hoc testing, in the sense that you don't get a free pass to just run stuff on the simulator.