Add rudimentary MUC support

I've made a start-bot-muc function for joining a bot to a chatroom.  There's a faked usage example in examples/.

This invoved making a bot/start-chat-muc function, and factoring some common code out of start-chat.  There's a minor API change in message->map in that because I want to return a message of the same type as that received, it doesn't make sense to convert the type from an enum to a string, so it's just left as the original object returned by getType.