Better default directories

Default config and data directories are now set according to where it
is installed:
  * /etc/unladen and /var/lib/unladen if installed in /usr/lib
    (distro-provided)
  * /usr/local/etc/unladen and /usr/local/lib/unladen if installed
    in /usr/local/lib (manual install)
  * ~/.unladen/etc and ~/.unladen/var otherwise (not installed)