feat(rt): make tokio runtime optional

A Cargo feature `runtime` is added, which is enabled by default, that
includes the following:

- The `client::HttpConnector`, which uses `tokio::net::TcpStream`.
- The `server::AddrStream`, which uses `tokio::net::TcpListener`.
- The `hyper::rt` module, which includes useful utilities to work with
  the runtime without needing to import `futures` or `tokio` explicity.

Disabling the feature removes many of these niceties, but allows people
to use hyper in environments that have an alternative runtime, without
needing to download an unused one.

src/server/tcp.rs

@@ -5,9 +5,8 @@ use std::time::Duration;
 
 use futures::{Async, Future, Poll, Stream};
 use futures_timer::Delay;
-//TODO: change to tokio_tcp::net::TcpListener
-use tokio::net::TcpListener;
-use tokio::reactor::Handle;
+use tokio_tcp::TcpListener;
+use tokio_reactor::Handle;
 
 use self::addr_stream::AddrStream;
 
@@ -170,7 +169,7 @@ mod addr_stream {
     use std::net::SocketAddr;
     use bytes::{Buf, BufMut};
     use futures::Poll;
-    use tokio::net::TcpStream;
+    use tokio_tcp::TcpStream;
     use tokio_io::{AsyncRead, AsyncWrite};