fix for pr2 transmission

- this patch is based on https://github.com/ros/urdfdom/commit/0915e8dcce3f37b21c5aad7b2a6b6ee7e17cf511, but updated some definition, because http://wiki.ros.org/urdf/XML/Transmission is not sufficient
- for example
-- https://github.com/PR2/pr2_common/blob/hydro-devel/pr2_description/urdf/upper_arm_v0/upper_arm.transmission.xacro
-- https://github.com/PR2/pr2_common/blob/hydro-devel/pr2_description/urdf/torso_v0/torso.transmission.xacro
-- https://github.com/PR2/pr2_common/blob/hydro-devel/pr2_description/urdf/shoulder_v0/shoulder.transmission.xacro
-- https://github.com/PR2/pr2_common/blob/hydro-devel/pr2_description/urdf/gripper_v0/gripper.transmission.xacro
-- https://github.com/PR2/pr2_common/blob/hydro-devel/pr2_description/urdf/forearm_v0/forearm.transmission.xacro
- #47 need to be modified
- this patch is requred for pr2_calibration package