New tmux/battery.py, using new battery_warningd.

tmux/battery.py
	-Rewrite the `tmux` battery-status script, which, like the old
	`battery_warningd`, relied on direct battery file access and
	replicated some of its code.
	-Simply import the new `battery_warningd`, and take advantage of
	its `_get_battery_status()`.