convert python dates into iso strings for javascript

iso dates py -> js

    def dto(self):
        """
        Data Transfer Object
        http://en.wikipedia.org/wiki/Data_transfer_object

        datetimes are serialized to http://www.ietf.org/rfc/rfc3339.txt
        see: http://stackoverflow.com/questions/455580/json-datetime-between-python-and-javascript/456032#456032
        """
        return dict(key=self.key().name(),
                    name=self.name,
                    created_at=self.created_at.strftime('%Y-%m-%dT%H:%M:%S'))