update model with gripper

- It seems original xacro file is not published in public repo, so update your xacro to generate this urdf and put baxter.urdf and baxter_gripper.urdf
- if we load this model in gazebo, the arm slowly moves down after launch and if I changed the weight of the gripper, the robot arms disappeared. it seems something wrong.....
https://github.com/RethinkRobotics/baxter_common/pull/62/files#diff-9b2bd50d5be98d59469020d9cd8ac8e9R623