Added more flexibility, improved support for scale handling

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.
Merge pull request #6 from Clstroud/master

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