Fix for pr2_calibration package to work

- this patch is based on 0915e8d, 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

Cc: @garaemon
> We shold not put pr2 specific things in the urdfdom
I agree with you, however PR2 requires several "extension" tags and old urdf parser could handle them because on groovy pr2 calibration worked. 

What is the best way to solve this issue?