Added options menu and menu transitions.

src/gstates/gstate.lua

@@ -20,13 +20,14 @@ local GameState = make_class(Drawable)
 
 function GameState:_init(name, index)
     Drawable._init(self)
-    self.name   = name
-    self.index  = index
+    self.name       = name
+    self.index      = index
+    self.next_state = self.index
 end
 
 
 function GameState:update(_)
-    return self.index
+    return self.next_state
 end