ref: https://stackoverflow.com/questions/4988286/what-difference-is-there-between-webclient-and-httpwebrequest-classes-in-net
WebClient 类更上层，比如得到http相应，对于的HttpWebRequest类的实现：
var response = http.GetResponse();
var stream = response.GetResponseStream();
var sr = new StreamReader(stream);
var content = sr.ReadToEnd();
对应的WebClient类：
var content = client.DownloadString("http://example.com");
Webclient没有timeout类，默认值是100s
