Added damagedState to WallSprite, PillboxSprite

Currently unused, marked with a TODO. Creates an 'unused field' warning
in each class, which should be OK.
Cleaned up ColorSet calculations

Makes getting sprite colors just a little less ugly.
Tank, Engineer, Pillbox, Base, Mine,
Added warning notes to PillboxSprite, WallSprite
Merge pull request #186 from BU-CS673/Aaron

Minor texture changes (damaged sprite cleanup)