Add a static fat binary for OS X

This includes both i386 and x86_64 builds within the same file. It is statically linked with libusb.

The build can be reproduced by uncommenting the indicated lines in the Makefile, otherwise by default it builds a dynamic linked binary. My understanding is that dynamic is fine for anybody building their own tools, but static is best for packaging downstream (e.g. in the Digispark Arduino app).