Fix homepage example to use index

Returning a model in `ApplicationRoute` might cause the application template to re-render in some cases (as mentioned [here](http://discuss.emberjs.com/t/getting-started-snippet-question/851)).

The application template should never re-render because it is appended to the body, therefore causing a duplicate template on re-render.

This PR also adds an `{{outlet}}`, and since some people are using this example as a starting point, having an outlet ready will make it easier for them to build on the example.