Hough circle detector

add code to detect circles based on hough transformation
* nodelet and executable are supported
* sample launch file (hough_circle_sample.launch)
* new message for 2D circles
* publish marker for image_view2

![screenshot from 2014-06-06 20 27 10](https://cloud.githubusercontent.com/assets/40454/3199710/0449cbaa-ed6e-11e3-80b9-046b01ddb912.png)