Throw an error when double init/shutdown
Throw an error when double init/shutdown

Currently, the rclnodejs doesn't reject the promise when it's initialized
two times, and does throw an error when it was shut twice.

To make these two behaviors consistently, this patch implements a strategy,
which will throw an error when init/shutdown are called continuously.

Fix #66

RobotWebTools/rclnodejs,minggangw/rclnodejs-1,minggangw/rclnodejs-1,RobotWebTools/rclnodejs,minggangw/rclnodejs-1,RobotWebTools/rclnodejs,RobotWebTools/rclnodejs,RobotWebTools/rclnodejs,minggangw/rclnodejs-1