Thermite Effectiveness Scaling with Volume

-The more thermite is applied to a wall, the faster it will burn
-Scales between approx 10-25 seconds
-Rwalls have a higher minimum threshold and burn time
-Applying thermite to a wall now leaves a visible stain on the wall

Conflicts:
	code/game/turfs/simulated/walls.dm
	icons/effects/effects.dmi
Added a sound effect to igniting thermite so it's not a silent stealthy move.
Thermiting walls leave admin prints