add plotter2d plugin

2d plotter on rviz using overlay

this topic retrieves `std_msgs/Float32`
http://youtu.be/l4Cmnf82_zs

![screenshot from 2014-05-05 16 12 43](https://cloud.githubusercontent.com/assets/40454/2875706/ac74a0ba-d424-11e3-9250-7d6527868f7e.png)