changed how ShopScreen gets its instance of shopItems
Merge branch 'towerdefense' of https://github.com/duke-compsci308-spring2013/vooga into towerdefense
added convenience methods to Pixmap and fixed bug in ShopScreen that prevented shopItems from registering mouse clicks
Merge branch 'towerdefense' of https://github.com/duke-compsci308-spring2013/vooga into towerdefense
Merge branch 'towerdefense' of https://github.com/duke-compsci308-spring2013/vooga into towerdefense