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`.

src/lib.rs

@@ -59,7 +59,7 @@
 //!         .expect("unable to bind TCP listener");
 //!
 //!     // Pull out a stream of sockets for incoming connections
-//!     let server = listener.incoming().for_each(|(sock, _)| {
+//!     let server = listener.incoming().for_each(|sock| {
 //!         // Split up the reading and writing parts of the
 //!         // socket.
 //!         let (reader, writer) = sock.split();