js-duk: Add a channel object to the message_received objects

    function handler(ev)
    {
        switch (ev.type) {
        case "message_received":
                msg = ev.data;
                ...
                mw.print("msg.channel.id: " + msg.channel.id);
                mw.print("msg.channel.name: " + msg.channel.name);
                mw.print("msg.channel.topic: " + msg.channel.topic);
                mw.print("msg.channel.soundproof: " + msg.channel.soundproof);
                ...
    }
    mw.onevent.push(handler);