Use postinstall to generate the JavaScript messages

[minggangw]

As the rclnodejs will generate the JavaScript messages based on the IDL files (.srv/.msg), the time consumption of first-time start up is usually over several seconds.

We want to move the generation of the messages to the stage of installing to accelerate the start up speed for the first time.

[qiuzhong]

@minggangw , does this mean that rclnodejs.init() will not generate these message files any more?

[minggangw]

During the installation (npm install), the message files will be created, and if rclnodejs.init() finds that the work has been done, they will not generated again which can accelerate the speed of start up.

@qiuzhong please verify this issue with #357 to check if the needed messages are generated after the execution of npm install. Thanks!

[qiuzhong]

Sure. I'm testing for it.

[qiuzhong]

Just tried #357 and it worked!