Refactor battery_warningd.py.

shell_scripts/battery_warningd.py
	-Remove `stdout` and `stderr` arguments from `subprocess.call()`
	calls; they were both incorrect (passing in `os.devnull`, a
	string, without opening it first) and unnecessary, so remove
	them entirely.
	-Refactor `_load_config()` and `_emit_warning()` along the lines
	of dependency injection.
	-Add expectation of a `sound` key in the config file, which
	specifies the path of the sound file (yet to be added).
Update battery_warningd documentation.

shell_scripts/battery_warningd.py
	-Update the docstrings in `battery_warningd` to reflect the
	changes introduced in 5dd5ab3.