imageView添加阴影和边框

注意：大量设置阴影会造成卡顿！！！

用上这句之后流畅度大大增加：imageV.layer.shouldRasterize = YES;

例：

        // 设置阴影

        imageV.layer.shadowOffset = CGSizeMake(4, 0);

        imageV.layer.shadowColor = [UIColor blackColor].CGColor;

        imageV.layer.shadowOpacity = 0.6;

        imageV.layer.shadowRadius = 3;

        imageV.layer.shouldRasterize = YES;