Massive refactoring

Moved all of the battery stuff into its own module.
No longer display seconds for remaining battery.

I find it a bit distracting.
Added notification support for the battery.

It successfully notifies us of AC adapter events. I've also written some
stuff for low battery status, but haven't tested it yet (I will when my
battery is low).

The logic for the low battery detection needs some work - right now it's
liable to give us several notifications as the estimated time remaining
fluctuates. We need to "debounce" the input.
Got rid of an awkward comma in the battery display