Implement battery_warning audio with pure Python.

res/battery_warning.wav
	-Change the notification sound `wav`.

shell_scripts/battery_warning_init.sh
	-Improve the regex used to match `battery_warningd.py` instances
	in `$(ps -ef)`.

shell_scripts/battery_warningd.py
	-Convert the code that plays the script's warning sound from a
	hackish `os.system()` call to an external utility (which
	frequently failed for obscure reasons) to a pure Python*
	solution using `pyglet`.

	* - Pure Python as far as I'm aware, and as far as it matters,
		anyway.