_isSafeMessage = function(e, settings) {
        if (settings.xdomain !== '*') {
            // If origin doesn't match our xdomain, return.
            if (!e.origin.match(new RegExp(settings.xdomain + '$'))) { return; }
        }

        return true;
    }
Check the message to make sure it comes from an acceptable xdomain.
Defaults to '*' but can be overriden in config.

@method _isSafeMessage
@param {Event} e The message event.
@param {Object} settings Configuration.