Added Mac OS X style framework as possible build target

This is against the 6.2.x branch.

I want to use Chipmunk as a dynamic library on Mac, not a static library. The best way to do this is as a .framwork. So this adds a new build target to the Xcode project so you can build as a framework.

There is a bit of detail work and I think others could benefit from this option, so I am contributing it. 

This will correctly bundle the header files in the framework (including the nested directory structure).
This also sets the install_name to use @rpath, following best modern (as of 10.5) conventions making it ready for bundling in .app bundles.

I think I bumped the min iOS version to 5.1 because you can't get modern arm64 setting it too low.