Refactored inset calculation into a separate method, altered sensitivity in determining what constitutes a transparent pixel (optional).

Separated the inset calculation as it alone was needed for my summer research project, but other changes I made in order to better support retina displays seemed worthy of contributing back.

Now an optional boolean flag can be passed to allow the image to be cropped based on whether pixels are semi-transparent. See the demo Xcode project for an example.

I also modified the style somewhat, which I normally would not do for open source code, but when I originally performed the modifications I hadn't considered the changes being substantial enough to contribute back. Hopefully they aren't significant enough to be bothersome.