main now calls ugrade() on all drawable objects. changed upgradeObject.update() to conform with base class update() so it gets called
explicitely default DrawableObject.toRemove to false even though its implicit

people were setting it to false in derived classes which was unnecessary
added new upgrade types and made them upgrade the kart so the kart doesnt have to know about different types. karts are encapsulated from stats but not parts yet (parts work as well as before though)
added new speed upgrade