Serialize to JSONP.

        :return string: serializaed JSONP

def serialize(self, content):
        """ Serialize to JSONP.

        :return string: serializaed JSONP

        """
        content = super(JSONPEmitter, self).serialize(content)
        callback = self.request.GET.get('callback', 'callback')
        return u'%s(%s)' % (callback, content)