Use awful's getdir() to handle XDG_CONFIG_HOME

For those of us who have `XDG_CONFIG_HOME` set the hard coded `~/.config` locations may be incorrect.  This change uses `awful`'s `getdir()` to access the [correct directory](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html).

There is a remote possibility for breakage should a current user have set `XDG_CONFIG_HOME`, but accepted that this repo is going to live in `~/.config/awesome`.  Not sure if, or how, that should be handled.

Thanks,

James