Fixed OS X install

When cx_freeze builds gmosh it copies several Python modules and libraries in the bin folder with gmosh itself. When these are not present in the same directory as gmosh, errors are thrown. I'm not sure why it worked on Linux. Perhaps because these modules were in the PATH somewhere. It may be worth using this fix for Linux too as as it is right now, an installation of Python 3.3 and/or a specific distro is likely required.

Anyway, this moves gmosh and its dependencies to its own folder in /usr/lib and places a symlink to gmosh itself in /usr/bin.
Additionally I included the correct Steam API library for OS X. It's a .dylib rather than a .so.
The libsteam.so check was removed. Again OS X uses .dylib for this but the location on this can greatly vary on where the user has placed Steam.app. It seems to work fine for me not matter where it is.
I've also added an uninstall script for OS X and added installation and compilation instructions to the READMEs and the HOW TO INSTALL.txt. They're really not that different to Linux.