当使用UIScrollView和其子类UITableView、UITextView之类的时候，可以简单的设置ScrollView的属性keyboardDismissMode，最低支持iOS7
typedef NS_ENUM(NSInteger, UIScrollViewKeyboardDismissMode) {
UIScrollViewKeyboardDismissModeNone,
UIScrollViewKeyboardDismissModeOnDrag, // dismisses the keyboard when a drag begins
UIScrollViewKeyboardDismissModeInteractive, // the keyboard follows the dragging touch off screen, and may be pulled upward again to cancel the dismiss
} NS_ENUM_AVAILABLE_IOS(7_0);
UIScrollViewKeyboardDismissModeOnDrag
ScrollView拖动的时候消失
UIScrollViewKeyboardDismissModeInteractive
交互式效果，拖动ScrollView可以使键盘移出屏幕范围，可取消
