split out kurt_gazebo_plugins into own package

moved to https://github.com/uos/diffdrive_gazebo_plugin

to update:
    perl -e 's/kurt_gazebo_plugins/diffdrive_gazebo_plugin/g' -pi $(find -name .git -prune -o -type f -print)
    perl -e 's/gazebo_ros_kurt/gazebo_ros_diffdrive_uos/g' -pi $(find -name .git -prune -o -type f -print)