Initial commit of KUKA KR120 support package

Added support for the KUKA KR120 R2500 PRO manipulator
Meshes are based on the Unigraphics files found at http://www.kuka-robotics.com/res/sps/8ba89f50-3ccd-4361-9b9a-60bdd0662ac6_KR120_R2500_pro.prt.zip
Note that the rotation directions of the KUKA manipulators are different than e.g. the ABB manipulator. For joint 1 this is implemented using <axis xyz="0 0 -1"/>.
KUKA is using A1, A2, etc for the joints. I have used joint_1, joint_2, .. to make the package consistent with support packages from the other manufactures, e.g. ABB. What do you suggest?