Gazebo and ros_control URDF Tags

Changes:

* Remove the inertial tag for the base link to fix the KDL warning: "The root link base has an inertia specified in the URDF, but KDL does not support a root link with an inertia. As a workaround, you can add an extra dummy link to your URDF."
* Add transmission tags for all of Baxter's joints - adds compatibility with ros_control and the Gazebo controllers
* Add the ros_control plugin for Gazebo
* Turn on collision checking between Baxter's torso and all other links in Gazebo