D-Bus support in contrib/battery-portable.lisp

I have added D-Bus support to the battery-portable contrib package. It polls the U-Power system for battery information. This is a workaround, of sorts, for my battery path disappearing in sysfs once I disconnect the line power. This results in me never knowing how much battery I have left, which defeats the purpose of the package.

There are some aspects of this code that I'm uncertain about, specifically the aesthetic appeal of the code. Also, I think whitespace might be messed up, but I'm not certain how to correct it inside of emacs, as I didn't do anything to change it in the first place.

"Why do we want this patch then? You make it sound awful!" Well, it works. Also, I'm open to critiques and requests for changes before the code is merged in, so this isn't me just dumping some one-off code.

Anyways, hopefully others think this will be useful, and can help critique or guide me on formatting and style.