scrollView有几个常用
@property(nonatomic) CGPoint contentOffset; // default CGPointZero
@property(nonatomic) CGSize contentSize; // default CGSizeZero
@property(nonatomic) UIEdgeInsets contentInset; // default UIEdgeInsetsZero. add additional scroll area around content
contentOffset设置scrollView滚动位置;
contentSize设置scrollView的内容尺寸"滚动范围";
contentInset设置scrollView的额外滚动范围;
注意：contentInset的值并不会影响到contentOffset和contentSize.
