add a Proxy type

Proxies can currently be configured to intercept HTTP, HTTPS, and all
requests. HTTPS tunneling is supported.

Closes #30

tests/proxy.rs

@@ -0,0 +1,42 @@
+extern crate reqwest;
+
+#[macro_use]
+mod support;
+
+#[test]
+fn test_http_proxy() {
+    let server = server! {
+        request: b"\
+            GET http://hyper.rs/prox HTTP/1.1\r\n\
+            Host: hyper.rs\r\n\
+            User-Agent: $USERAGENT\r\n\
+            Accept: */*\r\n\
+            Accept-Encoding: gzip\r\n\
+            \r\n\
+            ",
+        response: b"\
+            HTTP/1.1 200 OK\r\n\
+            Server: proxied\r\n\
+            Content-Length: 0\r\n\
+            \r\n\
+            "
+    };
+
+    let proxy = format!("http://{}", server.addr());
+
+    let url = "http://hyper.rs/prox";
+    let res = reqwest::Client::builder()
+        .unwrap()
+        .proxy(reqwest::Proxy::http(&proxy).unwrap())
+        .build()
+        .unwrap()
+        .get(url)
+        .unwrap()
+        .send()
+        .unwrap();
+
+    assert_eq!(res.url().as_str(), url);
+    assert_eq!(res.status(), reqwest::StatusCode::Ok);
+    assert_eq!(res.headers().get(),
+               Some(&reqwest::header::Server::new("proxied")));
+}