Fix PR2 calibration by using hacked version of urdf_parser_py by overwriting PYTHONPATH in the shell scripts.

* add pr2_urdf_parser_py directory by copying from https://github.com/ros/urdfdom/pull/55
* overwrite PYTHONPATH when running multi_step_cov_estimator.py to use urdf_parser_py under pr2_urdf_parser_py directory instead of official urdf_parser_py
* You can test calibration by https://drive.google.com/file/d/0B4bnoIcbRJVuVWVqT1ExR0hvcEE/view?usp=sharing