move <visual> from pr2.xml into pr2_visual.xml, change <map> for material to <material> directly. (ros ticket #368)

[ahendrix]

as a consequence of tear down, we need to:

• update urdf2gazebo
• update URDF parser
• Josh's visualization
• update wg_robot_descriptions

trac data:

• Owner: hsu
• Reporter: hsu
• Reported at: Tue Sep 30 13:15:43 2008
• URL: https://code.ros.org/trac/wg-ros-pkg/ticket/368

[ahendrix]

[hsu] after some discussion, we've decided not to do this afterall. visual will stay as part of robot description.