Add mob/living stat setter and observer

code/datums/observation/stat_set.dm

@@ -0,0 +1,24 @@
+//	Observer Pattern Implementation: Stat Set
+//		Registration type: /mob/living
+//
+//		Raised when: A /mob/living changes stat, using the set_stat() proc
+//
+//		Arguments that the called proc should expect:
+//			/mob/living/stat_mob: The mob whose stat changed
+//			/old_stat: Status before the change.
+//			/new_stat: Status after the change.
+
+GLOBAL_DATUM_INIT(stat_set_event, /decl/observ/stat_set, new)
+
+/decl/observ/stat_set
+	name = "Stat Set"
+	expected_type = /mob/living
+
+/****************
+* Stat Handling *
+****************/
+/mob/living/set_stat(var/new_stat)
+	var/old_stat = stat
+	. = ..()
+	if(stat != old_stat)
+		GLOB.stat_set_event.raise_event(src, old_stat, new_stat)