_makeMessage = function(id, messageType, message) {
        var bits = ['pym', id, messageType, message];

        return bits.join(MESSAGE_DELIMITER);
    }
Construct a message to send between frames.

NB: We use string-building here because JSON message passing is
not supported in all browsers.

@method _makeMessage
@param {String} id The unique id of the message recipient.
@param {String} messageType The type of message to send.
@param {String} message The message to send.