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.

examples/multi_server.rs

@@ -1,14 +1,10 @@
 #![deny(warnings)]
 extern crate hyper;
-extern crate futures;
 extern crate pretty_env_logger;
-extern crate tokio;
-
-use futures::{Future};
-use futures::future::{lazy};
 
 use hyper::{Body, Response, Server};
 use hyper::service::service_fn_ok;
+use hyper::rt::{self, Future};
 
 static INDEX1: &'static [u8] = b"The 1st service!";
 static INDEX2: &'static [u8] = b"The 2nd service!";
@@ -19,7 +15,7 @@ fn main() {
     let addr1 = ([127, 0, 0, 1], 1337).into();
     let addr2 = ([127, 0, 0, 1], 1338).into();
 
-    tokio::run(lazy(move || {
+    rt::run(rt::lazy(move || {
         let srv1 = Server::bind(&addr1)
             .serve(|| service_fn_ok(|_| Response::new(Body::from(INDEX1))))
             .map_err(|e| eprintln!("server 1 error: {}", e));
@@ -30,8 +26,8 @@ fn main() {
 
         println!("Listening on http://{} and http://{}", addr1, addr2);
 
-        tokio::spawn(srv1);
-        tokio::spawn(srv2);
+        rt::spawn(srv1);
+        rt::spawn(srv2);
 
         Ok(())
     }));