Push a notification to a specific pushed user.

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

def push_user(self, content, access_token, content_url=None):
        '''Push a notification to a specific pushed user.

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