Fixed crash in husky gazebo plugin.

Plugin now correctly disconnects callback in desctructor. Also got rid
of an unused member variable in header.
Kill ROS spin when sim object destroyed.
Merge remote-tracking branch 'origin/groovy' into groovy
Fixed a bug where the husky turns in gazebo, but the RVIZ model does not. Also added robot_description launch in the husky_base.launch file to enable compatibility with robot_state_publisher, joint_state_publisher and RVIZ robot model display.