Add tokio-tls echo example. (#541)

Based on the current example on the front page of tokio.rs.

tokio-tls/examples/echo.rs

@@ -0,0 +1,59 @@
+// A tiny async TLS echo server with Tokio
+extern crate native_tls;
+extern crate tokio;
+extern crate tokio_tls;
+
+use native_tls::Identity;
+use tokio::io;
+use tokio::net::TcpListener;
+use tokio::prelude::*;
+
+fn main() {
+    // Bind the server's socket
+    let addr = "127.0.0.1:12345".parse().unwrap();
+    let tcp = TcpListener::bind(&addr).unwrap();
+
+    // Create the TLS acceptor.
+    let der = include_bytes!("identity.p12");
+    let cert = Identity::from_pkcs12(der, "mypass").unwrap();
+    let tls_acceptor = tokio_tls::TlsAcceptor::from(
+        native_tls::TlsAcceptor::builder(cert).build().unwrap());
+
+    // Iterate incoming connections
+    let server = tcp.incoming().for_each(move |tcp| {
+
+        // Accept the TLS connection.
+        let tls_accept = tls_acceptor.accept(tcp)
+            .and_then(move |tls| {
+                // Split up the read and write halves
+                let (reader, writer) = tls.split();
+
+                // Copy the data back to the client
+                let conn = io::copy(reader, writer)
+                    // print what happened
+                    .map(|(n, _, _)| {
+                        println!("wrote {} bytes", n)
+                    })
+                    // Handle any errors
+                    .map_err(|err| {
+                        println!("IO error {:?}", err)
+                    });
+
+                // Spawn the future as a concurrent task
+                tokio::spawn(conn);
+
+                Ok(())
+            })
+            .map_err(|err| {
+                println!("TLS accept error: {:?}", err);
+            });
+        tokio::spawn(tls_accept);
+
+        Ok(())
+    }).map_err(|err| {
+        println!("server error {:?}", err);
+    });
+
+    // Start the runtime and spin up the server
+    tokio::run(server);
+}