Allow the client to explicitly control proxy

Fixes issue #109.

src/main/java/org/kohsuke/github/GitHubBuilder.java

@@ -6,6 +6,9 @@
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.Proxy;
+import java.net.URL;
 import java.util.Map;
 import java.util.Properties;
 
@@ -153,6 +156,19 @@ public GitHubBuilder withConnector(HttpConnector connector) {
         return this;
     }
 
+    /**
+     * Configures {@linkplain #withConnector(HttpConnector) connector}
+     * that uses HTTP library in JRE but use a specific proxy, instead of
+     * the system default one.
+     */
+    public GitHubBuilder withProxy(final Proxy p) {
+        return withConnector(new HttpConnector() {
+            public HttpURLConnection connect(URL url) throws IOException {
+                return (HttpURLConnection) url.openConnection(p);
+            }
+        });
+    }
+
     public GitHub build() throws IOException {
         return new GitHub(endpoint, user, oauthToken, password, connector);
     }