上下滑动TableView时让Navigation渐变，并且让headerView中的图片也跟着拖动放大

使用方法:

//init headview

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, 263)]; [imageView setImage:[UIImage imageNamed:@"mengyin"]]; header = [[WSHeaderView alloc]initWithTableViewWithHeaderImage:imageView.image withOTCoverHeight:imageView.frame.size.height withTableviewIsStyleGrouped:NO withTitle:@"detail"];