Added two diplomacy functions to lua

...and sneaked into AUTHORS

OpenRA.Game/Player.cs

@@ -111,6 +111,17 @@ namespace OpenRA
 			return p == null || Stances[p] == Stance.Ally;
 		}
 
+		public void SetStance(Player target, Stance s)
+		{
+			var oldStance = Stances[target];
+			Stances[target] = s;
+			target.Shroud.UpdatePlayerStance(World, this, oldStance, s);
+			Shroud.UpdatePlayerStance(World, target, oldStance, s);
+
+			foreach (var nsc in World.ActorsWithTrait<INotifyStanceChanged>())
+				nsc.Trait.StanceChanged(nsc.Actor, this, target, oldStance, s);
+		}
+
 		#region Scripting interface
 
 		Lazy<ScriptPlayerInterface> luaInterface;