Fix #253: Add proxy example

examples/proxy.php

@@ -0,0 +1,11 @@
+<?php
+require __DIR__ . '/vendor/autoload.php';
+
+use \Curl\Curl;
+
+$curl = new Curl();
+$curl->setOpt(CURLOPT_HTTPPROXYTUNNEL, 1);
+$curl->setOpt(CURLOPT_PROXY, 'someproxy.com:9999');
+$curl->setOpt(CURLOPT_PROXYUSERPWD, 'username:password');
+$curl->get('https://httpbin.org/get');
+var_dump($curl->response);