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/params.rs

@@ -2,7 +2,6 @@
 extern crate futures;
 extern crate hyper;
 extern crate pretty_env_logger;
-extern crate tokio;
 extern crate url;
 
 use futures::{future, Future, Stream};
@@ -93,5 +92,5 @@ fn main() {
         .serve(|| service_fn(param_example))
         .map_err(|e| eprintln!("server error: {}", e));
 
-    tokio::run(server);
+    hyper::rt::run(server);
 }