[jsk_rviz_plugins] ScreenshotListenerTool

ScreenshotListenerTool: A simple tool to listen to a service and save screenshot to specified file

You can save a image of rviz by simply calling ros service.

![screenshot_listener](https://cloud.githubusercontent.com/assets/40454/5563675/0f3acef6-8ecc-11e4-9fd0-1ebd6ca5cff9.png)