Fix ISpecialArmor for non-player entities

ISpecialArmor didn't seem to work for mobs at all, only players. This should fix that. One notable change is that I removed the armor inventory parameter (an ItemStack array) from ```ApplyArmor``` and replaced it by ```getEquipmentInSlot``` and ```setCurrentItemOrArmor```.
(As an added bonus I found and fixed some stuff in the documentation.)

Might cause issues with mods that blindly cast the EntityLivingBase to EntityPlayer.
May cause mobs with special armor to suffer of invincibility as well.