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/hello.rs

@@ -1,13 +1,10 @@
 #![deny(warnings)]
 extern crate hyper;
-extern crate futures;
 extern crate pretty_env_logger;
-extern crate tokio;
-
-use futures::Future;
 
 use hyper::{Body, Response, Server};
 use hyper::service::service_fn_ok;
+use hyper::rt::{self, Future};
 
 static PHRASE: &'static [u8] = b"Hello World!";
 
@@ -33,5 +30,5 @@ fn main() {
 
     println!("Listening on http://{}", addr);
 
-    tokio::run(server);
+    rt::run(server);
 }