added done callback to renderer start method.

The done callback existed in the ForceDirected layout start method, but no one passed that in. Maybe I don't understand how it's meant to be used? Or something was lost on the way with refactorings in springy? Anyway, with this change it's possible to call the Springy.Renderer start() method with a done callback. Please accept it, or document how it's meant to be used. Thanks ;-)