added makeCancelButton to ShopScreen
Merge branch 'towerdefense' of https://github.com/duke-compsci308-spring2013/vooga into towerdefense
created setCancelButtonVisibility in ShopScreen
Merge branch 'towerdefense' of https://github.com/duke-compsci308-spring2013/vooga into towerdefense
added setVisibilityOfCancelButton to the Controller