WebClient类和HttpWebRequest类

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