Replace route with Router::new().route() (#215)

This way there is now only one way to create a router:

```rust
use axum::{Router, handler::get};

let app = Router::new()
    .route("/foo", get(handler))
    .route("/foo", get(handler));
```

`nest` was changed in the same way:

```rust
use axum::Router;

let app = Router::new().nest("/foo", service);
```

examples/hello-world/src/main.rs

@@ -4,13 +4,13 @@
 //! cargo run -p example-hello-world
 //! ```
 
-use axum::{handler::get, route};
+use axum::{handler::get, Router};
 use std::net::SocketAddr;
 
 #[tokio::main]
 async fn main() {
     // build our application with a route
-    let app = route("/foo", get(handler));
+    let app = Router::new().route("/", get(handler));
 
     // run it
     let addr = SocketAddr::from(([127, 0, 0, 1], 3000));