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/rt.rs

@@ -0,0 +1,11 @@
+//! Default runtime
+//!
+//! By default, hyper includes the [tokio](https://tokio.rs) runtime. To ease
+//! using it, several types are re-exported here.
+//!
+//! The inclusion of a default runtime can be disabled by turning off hyper's
+//! `runtime` Cargo feature.
+
+pub use futures::{Future, Stream};
+pub use futures::future::{lazy, poll_fn};
+pub use tokio::{run, spawn};