Push a notification to a Pushed application.

        Param: content -> content of Pushed notification message
               content_url (optional) -> enrich message with URL
        Returns Shipment ID as string

def push_app(self, content, content_url=None):
        '''Push a notification to a Pushed application.

        Param: content -> content of Pushed notification message
               content_url (optional) -> enrich message with URL
        Returns Shipment ID as string
        '''
        parameters = {
            'app_key': self.app_key,
            'app_secret': self.app_secret
        }
        return self._push(content, 'app', parameters, content_url)