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/stream-buffered.rs

@@ -42,7 +42,7 @@ fn echo_server() {
     });
 
     let future = srv.incoming()
-                    .map(|s| s.0.split())
+                    .map(|s| s.split())
                     .map(|(a, b)| copy(a, b).map(|_| ()))
                     .buffered(10)
                     .take(2)