Retrieves the opens for this campaign.

def opens(self, date="", page=1, page_size=1000, order_field="date", order_direction="asc"):
        """Retrieves the opens for this campaign."""
        params = {
            "date": date,
            "page": page,
            "pagesize": page_size,
            "orderfield": order_field,
            "orderdirection": order_direction}
        response = self._get(self.uri_for("opens"), params=params)
        return json_to_py(response)