Catkin migration and some small fixes to urdf/xacro

These changes migrate the `gomtec_robot_description` package to the catkin build system, as well fix a small typo in the gripper xacro where the `mimic` tag referenced an non-existing joint.

If rosbuild compatibility is important, perhaps this should not be merged into `master`.

I also didn't add a metapackage, as I did not know what the preferred name would be.