add a Proxy type

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

Closes #30

src/into_url.rs

@@ -32,3 +32,7 @@ impl<'a> PolyfillTryInto for &'a String {
         Url::parse(self)
     }
 }
+
+pub fn to_uri(url: &Url) -> ::hyper::Uri {
+    url.as_str().parse().expect("a parsed Url should always be a valid Uri")
+}