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/global.rs

@@ -21,7 +21,7 @@ fn hammer() {
             let addr = t!(srv.local_addr());
             let mine = TcpStream::connect(&addr);
             let theirs = srv.incoming().into_future()
-                .map(|(s, _)| s.unwrap().0)
+                .map(|(s, _)| s.unwrap())
                 .map_err(|(s, _)| s);
             let (mine, theirs) = t!(mine.join(theirs).wait());