Implement transmission loading from URDF - Indigo

This PR supersedes #141. It's basically the same, but rebased on top of indigo-devel, hence with rosbuild artifacts removed.
This line causes pre-Indigo style transmission definitions to fail, rather than be deprecated as stated in ros-simulation/gazebo_ros_pkgs#184 and as done (correctly) in gazebo_ros_control/src/default_robot_hw_sim.cpp line 136 in ros-simulation/gazebo_ros_pkgs#185.
This fails currently because it continues out of the parseJoints() function without ever pushing the new Joint element below.  Until J Turtle, this should be a deprecation warning and not continue.