Added isValidArmor(ItemStack, int, EntityPlayer) in Item.java

I added this so that when the method is overriden, the player object may be accessed by a mod.
Changed method call in SlotArmor.java to pass in player.
To get player object, had to change visibility of thePlayer in ContainerPlayer to public.

Thanks for your time! If I messed anything up, I apologize in advance!