added a simple model controller plugin that uses a twist message

This controller can be used to move any entity on the XY plane as long as it has a high enough inertia. I use it when I have to spawn a large number of robots in simulation. I give the robots a cylindrical collision model, high inertia with a center of mass located close to the ground. This controller gives a fair amount of speedup in comparison to simulating the complete model and a differential drive controller.