Improve asset paths

This improves the asset paths in a couple ways

### Explicit namespacing

Instead of making the following GET requests for assets:

```
/assets/app.css
/assets/app.js
/assets/photo.jpg
```

You'll make these ones:

```
/assets/stylesheets/app.css
/assets/javascripts/app.js
/assets/images/photo.jpg
```

### Vendored assets

You can now throw CSS and JS into `/vendor/assets/stylesheets/` and `/vendor/assets/javascripts/`. You can load individual ones by making requests like `GET /assets/stylesheets/vendored.css`, or you can pick them all up automatically into your app bundle by adding `//= require_tree ../../../vendor/assets/javascripts` to your `app.js` and `//= require_tree ../../../vendor/assets/stylesheets` to your `app.css` (the generator does this for you automatically).

/cc @jessicard @fabianperez