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.

*edit:* Mods which use ApplyArmor with a custom inventory array or add additional armor slots by replacing the player's armor array will probably break in some way with the proposal.