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/form/src/main.rs

@@ -4,7 +4,7 @@
 //! cargo run -p example-form
 //! ```
 
-use axum::{extract::Form, handler::get, response::Html, route};
+use axum::{extract::Form, handler::get, response::Html, Router};
 use serde::Deserialize;
 use std::net::SocketAddr;
 
@@ -17,7 +17,7 @@ async fn main() {
     tracing_subscriber::fmt::init();
 
     // build our application with some routes
-    let app = route("/", get(show_form).post(accept_form));
+    let app = Router::new().route("/", get(show_form).post(accept_form));
 
     // run it with hyper
     let addr = SocketAddr::from(([127, 0, 0, 1], 3000));