rqt plugin for histogram visualization

* add new rqt plugin called HistogramPlot
* 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

sample code as a data provider:

```python
#!/usr/bin/env python

import rospy
from posedetection_msgs.msg import Feature0D
import numpy
rospy.init_node("histogram_test")

p = rospy.Publisher("histogram", Feature0D)
r = rospy.Rate(1)
while not rospy.is_shutdown():
  msg = Feature0D()
  msg.descriptors = numpy.random.randn(128)
  p.publish(msg)
  r.sleep()
```
![screenshot_from_2014-06-05 01 37 19](https://cloud.githubusercontent.com/assets/40454/3176610/d69fea30-ec06-11e3-900c-1bff593826e2.png)