rviz plugin for Overlay Camera

rviz plugin: OverlayCamera. it's an overlay version of rviz::Camera rather than rviz::Image.

You can see tf frame overlay.
![screenshot from 2014-06-24 14 52 18](https://cloud.githubusercontent.com/assets/40454/3367640/9dd973ca-fb64-11e3-8f57-f3a1bc96525a.png)