Gadgets now effect the player's skill level and cargo slots and difficulty effects players skill level.
Modified player.getEffectiveSkill so that now difficulty can effect the player's skill and if the player has certain gadgets, their respective skill levels are raised.
Added a ShipCargo class that extends Cargo. SpaceShip's cargo is a ShipCargo. The ShipCargo will increase the the value returned by getMaxCapacity().