Adding support for OSX

Adding OSX support, along with instructions on how to build the required shared library.  Confirmed to work on OSX 10.9 (Mavericks).

I added the shared library build instructions in its own README-OSX.txt file.  I don't know if this approach is standard procedure or not.  If you would prefer that I place the documentation somewhere else, please let me know and I will do so.

Cheers