a script to ping to check existence of node

`rosping_existence.py` is a script to check existence of node like `rosnode ping`.
The script publishes the existence to '/diagnostics' and you can visualize it via robot_monitor.

and the script enumerates the nodes from launch file like:
```
rosrun jsk_topic_tools rosping_existence.py $(rospack find jsk_pr2_startup)/pr2.launch
```

![screenshot_from_2014-07-15 01 50 27](https://cloud.githubusercontent.com/assets/40454/3573881/4f342008-0b77-11e4-82b9-54ad3cd22851.png)