Added in code for Tech Levels using prereqs

and fixed up some kinks of the old system

OpenRA.Game/Map/Map.cs

@@ -31,6 +31,7 @@ namespace OpenRA
 		public bool? AllyBuildRadius;
 		public bool? FragileAlliances;
 		public int? StartingCash;
+		public string TechLevel;
 		public bool ConfigurableStartingUnits = true;
 		public string[] Difficulties = { };