Fixed 'No Actuator Element' errors on launch with Gazebo 1.9

Changed transmission xacros to include a hardwareInterface tag within each actuator tag according to gazebo <transmission> specs found at http://gazebosim.org/wiki/Tutorials/1.9/ROS_Control_with_Gazebo. This gets rid of 'No Actuator Element' errors on launch.  Left hardwareInterface tags in joint tags for the time being as they don't seem to produce errors'.