rqt plugin for histogram visualization

* add new rqt plugin called rqt_histogram_plot
* usage `rosrun jsk_rqt_plugins rqt_histogram_plot /topic/to/float/array`

this program can visualize array of the number such as `posedetection_msgs/Feature0D/descriptors`.

see https://github.com/jsk-ros-pkg/jsk_visualization/issues/78

![screenshot_from_2014-06-05 01 37 19](https://cloud.githubusercontent.com/assets/40454/3176610/d69fea30-ec06-11e3-900c-1bff593826e2.png)