Change net::Incoming signature to match std. (#89)

std's `Incoming` iterator yields `TcpStream` instances. This patch
updates the `Incoming` future to match this signature.

This changes the yielded value from `(TcpStream, SocketAddr)` ->
`TcpStream`.

tests/buffered.rs

@@ -46,7 +46,7 @@ fn echo_server() {
         (expected, t2)
     });
 
-    let clients = srv.incoming().take(2).map(|e| e.0).collect();
+    let clients = srv.incoming().take(2).collect();
     let copied = clients.and_then(|clients| {
         let mut clients = clients.into_iter();
         let a = BufReader::new(clients.next().unwrap());