Default webview

Hey,

As I've been getting my head around Calatrava, I found it quite counter-intuitive that the iOS apps weren't showing the shell views if I didn't provide a native implementation. At a bare minimum, it required that I create a subclass of `WebViewController` and set the `pageName` to the same as my shell view. This seemed to go against the idea that at a minimum you could just build a shell, and replace the views piecemeal.

I had a look and realised it wouldn't be much work to change Calatrava's page retrieval/creation logic to default to using a `WebViewController` instance (with the `pageName` set) if no custom ViewController could be found.

What do you think?

Context: https://groups.google.com/d/topic/calatrava-mobile/dZnDmBJ7WGY/discussion

Cheers,
James