Retrieve one task, discriminated by id.

        :param: task_id: the task id

        :returns: a zobjects.Task object ;
                  if no task is matching, returns None.

def get_task(self, task_id):
        """Retrieve one task, discriminated by id.

        :param: task_id: the task id

        :returns: a zobjects.Task object ;
                  if no task is matching, returns None.
        """
        task = self.request_single('GetTask', {'id': task_id})

        if task:
            return zobjects.Task.from_dict(task)
        else:
            return None