compute color histogram of sensor_msgs::Image

add a program to compute color histogram of RGB image
* both of 
nodelet and executable are supported
* work with image_view2
* work with rqt_histogram_plot of jsk_rqt_plugins

![screenshot_from_2014-06-06 03 08 33](https://cloud.githubusercontent.com/assets/40454/3191613/132007c0-ecdd-11e3-9284-ee18e992ad1c.png)