LGHDownloadManager
简易断点下载，简易api. Simple breakpoint download, simple api. API:
- (instancetype)sharedManager; //开始下载或继续下载
- (void)startDownloadingWithURL:(NSURL*)url; //暂停下载
- (void)pause; //恢复下载
- (void)resumeDownload; //获取下载进度
- (void)getDownloadDataPercentWithBlock:(DownloadBlock)block; //清空下载目录
- (void)clearCache;