两种弹出模式：alert,sheet
###1.alert（中间弹出）
HUDatePicerView *he = [[HUDatePicerView alloc] initWithDateFormat:nil
mode:HUDatePickerModeDate maxDate:nil minDate:nil style:alert];
he.delegate = self;
[self.view addSubview:he];
###2.sheet
HUDatePicerView *he = [[HUDatePicerView alloc] initWithDateFormat:nil
mode:HUDatePickerModeDate maxDate:nil minDate:nil style:sheet];
he.delegate = self;
[self.view addSubview:he];