Add Mac i386 dylibs

This was built and tested on Mac OS X 10.5, and should work on any newer version. I wasn't able to install a 32-bit ruby on a newer version of OS X to test for sure though.

My OS X 10.6 box is on the fritz right now, so I can't compile a reasonably compatible 64-bit dylib. (64-bit support on 10.5 is very iffy, and I don't have 64-bit 10.5 hardware.) I'll submit another PR for that when I get the chance.