iOS: Allow external modules to use ARC

The original feature request was for ARC in all of TiMob codebase. This is less ambitious, and just to allow modules that use ARC to interact with Titanium.

Automatic Reference Counting (ARC) for Objective-C makes memory management the job of the compiler. By enabling ARC with the new Apple LLVM compiler, you will never need to type retain or release again, dramatically simplifying the development process, while reducing crashes and memory leaks. The compiler has a complete understanding of your objects, and releases each object the instant it is no longer used, so apps run as fast as ever, with predictable, smooth performance.

More info here.