python: append paths from PYTHON<VERSION>PATH env var to sys.path

Use of this (admittedly non-standard) variable enables a safer way to
extend Python interpreters with modules that are installed in a nix
profile.

Instead of the unsafe

  export PYTHONPATH=~/.nix-profile/lib/python2.7/site-packages

which probably breaks if you start python3, you can now

  export PYTHON27PATH=~/.nix-profile/lib/python2.7/site-packages
  export PYTHON33PATH=~/.nix-profile/lib/python3.3/site-packages

and there will be no interference between python2.7 and python3.3.