SDWebImage中的一些参数：
*SDWebImageRetryFailed = 1<< 0, 默认选项，失败后重试
*SDWebImageLowPriority = 1<< 1, 使用低优先级
*SDWebImageCacheMemoryOnly = 1<< 2, 仅仅使用内存缓存
*SDWebImageProgressiveDownload = 1<< 3, 显示现在进度
*SDWebImageRefreshCached = 1<< 4, 刷新缓存
*SDWebImageContinueInBackground =1 << 5, 后台继续下载图像
*SDWebImageHandleCookies = 1<< 6, 处理Cookie
*SDWebImageAllowInvalidSSLCertificates= 1 << 7, 允许无效的SSL验证
*SDWebImageHighPriority = 1<< 8, 高优先级
*SDWebImageDelayPlaceholder = 1<< 9 延迟显示占位图片