virtualenvwrapper support, PYTHONPATH

Set sane defaults for `$WORKON_HOME`, `$PROJECT_HOME`, and
`$VIRTUALENVWRAPPER_HOOK_DIR`. But created function and hook to
automatically detect my meta project dir when changin directories and
change these directories accordingly so that virtualenvwrapper can find
my per-project venvs.

Also updated and filled out PYTHONPATH.