Added support for monarch.post()

test/data/screen1.gui_script

@@ -7,6 +7,7 @@ function final(self)
 end
 
 function on_message(self, message_id, message, sender)
-	-- Add message-handling code here
-	-- Remove this function if not needed
+	if message_id == hash("foobar") then
+		_G.screen1_foobar = message or true
+	end
 end