Added the possibility to add a chain of request handlers for a certain route

As described in https://github.com/dartist/express/issues/16 I added the possibility to register a chain of request handlers. This allows the developer to use little pieces of middleware.

```Dart
// Controllers
HomeController home = new HomeController();

// Middlewares
AuthMiddleware auth = new AuthMiddleware();

var app = new Express();
// The old way but still working with this update
app.get('/', home.index);

// The way it could be done with middleware
app.get('/users').then(auth.isSignedIn).then(home.users);
```

Because if the changes, it's not possible to chain them like this
```Dart
var app = new Express();
app.get('/', home.index)
     ..get('/users', home.users);
```

If this pull request is accepted, I will update the README documentation.