Rewrite the battery warning script.

shell_scripts/battery_warningd.py
	-The previous battery-warning script was mysteriously silently
	failing. Was also unportable, because it read specific
	battery-related files on my Carbon's filesystem, and thus would
	usually fail on other computers (which typically don't have
	them).
	-Rewrite in full: functionality remains much the same.
Replace JSON battery-warning config with Yaml.

res/battery_warning/config.(json -> yml)
	-Convert the JSON config file to a more human-friendly format.
Remove battery_warning_init.sh.

shell_scripts/battery_warning_init.sh
	-Remove the `battery_warning_init.sh`, which was called by
	`cron` minutely and checked if `battery_warningd.py` was
	running; if not, it started it.
	-Opting for a solution that kicks off the battery-checker on
	boot from `/etc/rc.local`, which is cleaner (also, there isn't
	any danger of the checker dying at some point during execution).
Thoroughly document battery_warningd.py.

shell_scripts/battery_warningd.py
	-Add module header/function docstrings.
Merge branch 'feature/new_battery_script' into develop

shell_scripts/battery_warningd.py, res/battery_warning/
	-Rewrite my battery-warning script, to hopefully solve
	mysterious failures to emit warnings and definitely resolve
	unportability.