/etc/profile: export PYTHON<VERSION>PATH env vars

Export PYTHON{26,27,32,33}PATH environment variables, similarly to how
we do it with PATH, PERL5LIB etc..

With the recently changed recursive-pth-loader in nixpkgs,
pkgs.pythonFull and pkgs.ipython (or any python interpreter that uses
recursive-pth-loader) will automatically pick up python modules
installed in the system path, the default profile and the user profile.