feat(TU20): Implement more of la's TU21 patch file

Forgot to include changes in HellBiome.cpp and EatTileGoal.cpp

TU20:
1. Skeletons & Wither Skeletons now rarely spawn anywhere in the Nether

Minecraft.World/HellBiome.cpp

@@ -14,4 +14,5 @@ HellBiome::HellBiome(int id) : Biome(id)
 	enemies.push_back(new MobSpawnerData(eTYPE_GHAST, 50, 4, 4));
 	enemies.push_back(new MobSpawnerData(eTYPE_PIGZOMBIE, 100, 4, 4));
 	enemies.push_back(new MobSpawnerData(eTYPE_LAVASLIME, 1, 4, 4));
+	enemies.push_back(new MobSpawnerData(eTYPE_SKELETON, 1, 4, 4));
 }