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/lib.rs

@@ -18,18 +18,20 @@
 
 extern crate bytes;
 #[macro_use] extern crate futures;
-extern crate futures_cpupool;
+#[cfg(feature = "runtime")] extern crate futures_cpupool;
 extern crate futures_timer;
 extern crate h2;
 extern crate http;
 extern crate httparse;
 extern crate iovec;
 #[macro_use] extern crate log;
-extern crate net2;
+#[cfg(feature = "runtime")] extern crate net2;
 extern crate time;
-extern crate tokio;
-extern crate tokio_executor;
+#[cfg(feature = "runtime")] extern crate tokio;
+#[cfg(feature = "runtime")] extern crate tokio_executor;
 #[macro_use] extern crate tokio_io;
+#[cfg(feature = "runtime")] extern crate tokio_reactor;
+#[cfg(feature = "runtime")] extern crate tokio_tcp;
 extern crate want;
 
 #[cfg(all(test, feature = "nightly"))]
@@ -62,3 +64,4 @@ mod headers;
 mod proto;
 pub mod server;
 pub mod service;
+#[cfg(feature = "runtime")] pub mod rt;