new rviz plugin to display overlay text

* new plugin to display overlay text on rviz. see the movie and image for detail
* new msg file for the overlay text display.

http://youtu.be/bF7wcuy6m64

![screenshot from 2014-05-04 23 08 25](https://cloud.githubusercontent.com/assets/40454/2873181/be9dad2a-d395-11e3-99b7-f9becd74f121.png)