[robots] Add PR2 robot.

python/example_robot_data/robots_loader.py

robots/pr2_description/meshes/base_v0/.gitignore

robots/pr2_description/meshes/base_v0/base.stl

robots/pr2_description/meshes/base_v0/base_L.stl

robots/pr2_description/meshes/base_v0/caster.stl

robots/pr2_description/meshes/base_v0/caster_L.stl

robots/pr2_description/meshes/base_v0/pr2_wheel.stl

robots/pr2_description/meshes/base_v0/wheel.stl

robots/pr2_description/meshes/forearm_v0/.gitignore

robots/pr2_description/meshes/forearm_v0/forearm.jpg

robots/pr2_description/meshes/forearm_v0/forearm.stl

robots/pr2_description/meshes/forearm_v0/wrist_flex.stl

robots/pr2_description/meshes/forearm_v0/wrist_roll.stl

robots/pr2_description/meshes/forearm_v0/wrist_roll_L.stl

robots/pr2_description/meshes/gripper_v0/.gitignore

robots/pr2_description/meshes/gripper_v0/gripper_palm.stl

robots/pr2_description/meshes/gripper_v0/l_finger.stl

robots/pr2_description/meshes/gripper_v0/l_finger_tip.stl

robots/pr2_description/meshes/head_v0/.gitignore

robots/pr2_description/meshes/head_v0/head_pan.stl