Make pyxtrlock conform with the XDG specification

According to the [XDG Base Directory Specification][1], XDG_CONFIG_HOME
is not always ~/.config. It can depend on the  environment variable
$XDG_CONFIG_HOME. Therefor use the right directory with the help of pyxdg.

  [1]: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html