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/multipart-form/src/main.rs

@@ -8,7 +8,7 @@ use axum::{
     extract::{ContentLengthLimit, Multipart},
     handler::get,
     response::Html,
-    route,
+    Router,
 };
 use std::net::SocketAddr;
 
@@ -21,7 +21,8 @@ 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))
         .layer(tower_http::trace::TraceLayer::new_for_http());
 
     // run it with hyper