Allow multiple envs in ~/.emacs.d/python-environments/

Difference to the current master is that with this patch, `~/.emacs.d/python-environments/ROOT-NAME` is the path to virtualenv rather than `~/.emacs.d/python-environment/`.  Emacs packages using python-environment.el should put env in `~/.emacs.d/python-environments/SOME-NAME` or just default location `~/.emacs.d/python-environments/default`.  This way, user just have to change `python-environment-directory` to change path to all virtualenv.

This is 90% in and final concern before the first release.  I am trying to think about demerit. If I can't find a big one, then this is in.