/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.

Will this ruin purity?
- This will not affect nixpkgs builders because they don't source
  /etc/profile.
- It will not affect custom python environments and wrapped python
  programs either, unless the wrappers use pkgs.pythonFull instead of
  pkgs.python. And even if they to use pkgs.pythonFull, dependencies
  should always be listed *first* in PYTHONPATH, so that any extra
  modules that might be made available to the program is either
  completely optional (e.g. extra python modules for ipython) or simply
  ignored (most wrapped python programs with fixed number of
  dependencies).