Add normal diplay

This will display normal using receiveing PointCloud2 topic (PointXYZRGBNormal is assumed.).
I implemented the program which publish MarkerArray before, but this time, I wrote this in rviz_plugins.(see jsk-ros-pkg/jsk_recognition#60)

I implemented this for tmp normal display code, but
I would like to and should integrate this normal display to pointcloud2 display in future.