Add HTTP proxy support in forward proxy (#745)

* Add HTTP proxy support in forward proxy

Signed-off-by: noisyfox <timemanager.rick@gmail.com>

* Typo

* Throw exception if forward proxy not supported.

shadowsocks-csharp/Model/ProxyConfig.cs

@@ -1,20 +1,22 @@
-using Shadowsocks.View;
-using System;
-using System.Drawing;
-using System.Windows.Forms;
+using System;
 
 namespace Shadowsocks.Model
 {
     [Serializable]
     public class ProxyConfig
     {
+        public const int PROXY_SOCKS5 = 0;
+        public const int PROXY_HTTP = 1;
+
         public bool useProxy;
+        public int proxyType;
         public string proxyServer;
         public int proxyPort;
 
         public ProxyConfig()
         {
             useProxy = false;
+            proxyType = PROXY_SOCKS5;
             proxyServer = "";
             proxyPort = 0;
         }