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();
    }
}
```

src/routing.rs

@@ -770,12 +770,16 @@ where
 
 fn strip_prefix(uri: &Uri, prefix: &str) -> Uri {
     let path_and_query = if let Some(path_and_query) = uri.path_and_query() {
-        let new_path = if let Some(path) = path_and_query.path().strip_prefix(prefix) {
+        let mut new_path = if let Some(path) = path_and_query.path().strip_prefix(prefix) {
             path
         } else {
             path_and_query.path()
         };
 
+        if new_path.is_empty() {
+            new_path = "/";
+        }
+
         if let Some(query) = path_and_query.query() {
             Some(
                 format!("{}?{}", new_path, query)