allow only one game perchat

bot.py

@@ -76,6 +76,16 @@ def new_game(bot, update):
 
     else:
 
+        try:
+            _game = gm.chatid_games[chat_id][-1]
+        except (KeyError, IndexError):
+            pass
+        else:
+            send_async(bot, chat_id,
+                       text=_("There is already a game running in this chat. Join the "
+                              "game with /join"))
+            return
+
         if update.message.chat_id in gm.remind_dict:
             for user in gm.remind_dict[update.message.chat_id]:
                 send_async(bot,