ARC-safe memory management

I updated your project to work for both ARC and non-ARC.  I'm using the macros from https://github.com/kstenerud/ARCSafe-MemManagement

It was pretty easy to convert over.  Please feel free to integrate my changes :)  I can assign the copyright in my changes to you so the licensing stays simple.  The one file I pulled from ARCSafe has a similar license to yours.

Thanks for putting up your code. Very useful!

  - mang