Add daemonized batteryWarning script.

Add:
	shell_scripts/batteryWarningInit.zsh
		-add script to be executed as a cronjob, which initializes
		batteryWarningd.zsh if it isn't already running.

Mod:
	shell_scripts/batteryWarning.sh -> batteryWarningd.zsh
		-convert to zsh script, as most of my system-administration
		shell-scripts use zsh.
		-append d to filename, to convey that it's a daemon.

	shell_scripts/batteryWarningd.zsh
		-convert from a script that executed only once to a daemon that relies
		on an internal, infinite loop that runs at intervals of 5 seconds and
		flashes battery-level warnings as appropriate. The previous iteration of
		the script was somewhat unreliable, as it was cron'd to run every minute
		and could potentially skip a battery level (if it decreased by a percent
		during that minute), and would consequently fail to send a notification
		if that level was critical.