URL for views as a flat attribute

The url of an html fragment can be set through the options of the view, but there's no reason why it shouldn't be able to be set directly, as part of the `extend` method: 

```
var view = APP.View.extend({
    options: {}, // this is where 'url' has to live currently...
    url: "assets/html/fragment.html"
});
```

This follows the same convention of Model/Collection in regular Backbone.js
