Update examples to use new Tokio (#316)

The old `tokio-core` crate is deprecated in favour of the `tokio` crate,
which also provides the new multithreaded Tokio runtime. Although `h2`
is generic over the runtime, the examples do depend on `tokio`. 

This branch update the example code to use the new `tokio` library
rather than `tokio-core`. It also updates the examples in `RustDoc`.

For the most part, this was pretty trivial --- simply replacing
`handle.spawn(...)` with `tokio::spawn(...)` and `core.run(...)` with
`tokio::run(...)`. There were a couple of cases where error and item
types from spawned futures had to be mapped to `(), ()`. Alternatively,
this could have been avoided by using the single-threaded runtime and
calling `block_on` instead to await the value of those futures, but I
thought it was better to reduce the amount of tokio-specific code in the
examples.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/server.rs

@@ -67,27 +67,23 @@
 //! extern crate futures;
 //! extern crate h2;
 //! extern crate http;
-//! extern crate tokio_core;
+//! extern crate tokio;
 //!
 //! use futures::{Future, Stream};
 //! # use futures::future::ok;
 //! use h2::server;
 //! use http::{Response, StatusCode};
-//! use tokio_core::reactor;
-//! use tokio_core::net::TcpListener;
+//! use tokio::net::TcpListener;
 //!
 //! pub fn main () {
-//!     let mut core = reactor::Core::new().unwrap();
-//!     let handle = core.handle();
-//!
 //!     let addr = "127.0.0.1:5928".parse().unwrap();
-//!     let listener = TcpListener::bind(&addr, &handle).unwrap();
+//!     let listener = TcpListener::bind(&addr,).unwrap();
 //!
-//!     core.run({
+//!     tokio::run({
 //!         // Accept all incoming TCP connections.
-//!         listener.incoming().for_each(move |(socket, _)| {
+//!         listener.incoming().for_each(move |socket| {
 //!             // Spawn a new task to process each connection.
-//!             handle.spawn({
+//!             tokio::spawn({
 //!                 // Start the HTTP/2.0 connection handshake
 //!                 server::handshake(socket)
 //!                     .and_then(|h2| {
@@ -114,8 +110,9 @@
 //!
 //!             Ok(())
 //!         })
-//!         # .select(ok(()))
-//!     }).ok().expect("failed to run HTTP/2.0 server");
+//!         .map_err(|e| panic!("failed to run HTTP/2.0 server: {:?}", e))
+//!  #      .select(ok(())).map(|_|()).map_err(|_|())
+//!     });
 //! }
 //! ```
 //!