修改cardGameViewController为抽象类，添加PlayingCardGameViewController实现了该抽象类
rename CardMatchingGame class to PlayingCardGame class
add SetCard class
modify SetCard class
重构代码，分离CardGame和PlayingCardGame 以及 CardGameViewController和PlayingCardGameViewController
完成setCardGameViewController基本功能
添加setCardGameViewController中对于选中的卡片的显示
add UITabBarController
完成assignment3基础部分
add tab bar item icon
Merge branch 'assignment3'