Add ability to create scene primitives like sphere/cylinder/capsule/cube etc

[zhangfq-chemistry]

Would you like to add some primitives including sphere/cylinder/capsule/polygons/circle/cone to this excellent project? I want to use them to build chemistry molecule.

[nmwsharp]

This would be a great feature! (Though I don't have time to immediately start working on it.)

A good reference is http://wiki.ros.org/rviz/DisplayTypes/Marker (from this twitter thread).