Add support for websockets (#3)

Basically a copy/paste of whats in warp.

Example usage:

```rust
use tower_web::{prelude::*, ws::{ws, WebSocket}};

let app = route("/ws", ws(handle_socket));

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = msg.unwrap();
        socket.send(msg).await.unwrap();
    }
}
```

examples/websocket.rs

@@ -0,0 +1,63 @@
+//! Example websocket server.
+//!
+//! Run with
+//!
+//! ```
+//! RUST_LOG=tower_http=debug,key_value_store=trace \
+//!     cargo run \
+//!     --features ws \
+//!     --example websocket
+//! ```
+
+use http::StatusCode;
+use hyper::Server;
+use std::net::SocketAddr;
+use tower::make::Shared;
+use tower_http::{
+    services::ServeDir,
+    trace::{DefaultMakeSpan, TraceLayer},
+};
+use tower_web::{
+    prelude::*,
+    routing::nest,
+    service::ServiceExt,
+    ws::{ws, Message, WebSocket},
+};
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt::init();
+
+    // build our application with some routes
+    let app = nest(
+        "/",
+        ServeDir::new("examples/websocket")
+            .append_index_html_on_directories(true)
+            .handle_error(|error| (StatusCode::INTERNAL_SERVER_ERROR, error.to_string())),
+    )
+    // routes are matched from bottom to top, so we have to put `nest` at the
+    // top since it matches all routes
+    .route("/ws", ws(handle_socket))
+    // logging so we can see whats going on
+    .layer(
+        TraceLayer::new_for_http().make_span_with(DefaultMakeSpan::default().include_headers(true)),
+    );
+
+    // run it with hyper
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    let server = Server::bind(&addr).serve(Shared::new(app));
+    server.await.unwrap();
+}
+
+async fn handle_socket(mut socket: WebSocket) {
+    if let Some(msg) = socket.recv().await {
+        let msg = msg.unwrap();
+        println!("Client says: {:?}", msg);
+    }
+
+    loop {
+        socket.send(Message::text("Hi!")).await.unwrap();
+        tokio::time::sleep(std::time::Duration::from_secs(3)).await;
+    }
+}

examples/websocket/index.html

@@ -0,0 +1 @@
+<script src='script.js'></script>

examples/websocket/script.js

@@ -0,0 +1,9 @@
+const socket = new WebSocket('ws://localhost:3000/ws');
+
+socket.addEventListener('open', function (event) {
+    socket.send('Hello Server!');
+});
+
+socket.addEventListener('message', function (event) {
+    console.log('Message from server ', event.data);
+});