allow groupings to not have pages

Sometimes there's nothing really to go on the grouping page (See App Internals/Views & Bindings), so allow it to just have a `<span>` and not have a page at all:

![groupings](https://cloud.githubusercontent.com/assets/2231765/2744292/71579e5a-c723-11e3-8338-2f37697f63c9.gif)


I'll make Views and Bindings use this once its merged.