实现本地程序从远程服务器下载文件。保存到本地。同时，具备断点续传的功能。可以在下载一部分的情况下，再次下载从断点处继续下载。-Implement a local program to download files a remote server. Saved locally. At the same time, with the functions of HTTP. You can download a part in the case, once again download continu