AI Core Update

Changes layout of the AI core, see picture in PR.
Moves APCs for core and upload to the backs of the rooms.
Critical APCs can somewhat resist EMP now.
Gives turrets a ten second timeout until they go underneath the turret covers, unless turned off manually or if they lose power.
Adds a new type of turret specifically for the AI upload/core.  This type has more health, has a 1/3 chance to ignore EMP, and fires xray lasers, to counter ablative armor.
A new material called Durasteel has been added.  It is made from Plasteel and diamonds, and boasts more toughness than Plasteel alone, as well as reflectivity.  The inner walls of the AI core are made out of this new material, which makes emitters less helpful.
Emitters can be damaged by projectiles, and will explode if too much damage is sustained.  Shooting a Durasteel wall will likely cause you to lose the emitter.
New AI core turrets turn green when on lethal, to show that xray is coming.

code/modules/mining/alloys.dm

@@ -8,6 +8,17 @@
 	var/product
 	var/metaltag
 
+/datum/alloy/durasteel
+	metaltag = "durasteel"
+	requires = list(
+		"diamond" = 1,
+		"platinum" = 1,
+		"carbon" = 2,
+		"hematite" = 2
+		)
+	product_mod = 0.3
+	product = /obj/item/stack/material/durasteel
+
 /datum/alloy/plasteel
 	metaltag = "plasteel"
 	requires = list(
@@ -32,4 +43,4 @@
 		"platinum" = 1,
 		"sand" = 2
 		)
-	product = /obj/item/stack/material/glass/phoronglass
+	product = /obj/item/stack/material/glass/phoronglass