Reads in sdf worlds and natively generates 3D visualizer.

The addRobotFromSDF.m command works with basic functionality (does not yet support include which is important in many of the files that we will be supplied) in of itself. The file runCinderBlocks.m runs into errors when executing constructVisualizer on the resultant object. This is because a struct is added in model.body element which cannot be solved into something a visualizer can use. This change occurs in removeFixedJoints in RigidBodyManipulator.m.