Add a `cardNavigationBarClass` configuration option

Again me with a configuration option ^^

Apple introduce in iOS5 `initWithNavigationBarClass:toolbarClass:`, a new `UINavigationController` method to properly customize the navigation bar used. 

I think it's important that we can control this aspect of the design. I needed that, to achieve taller cards, like those one.

![Capture d e cran du Simulateur iOS 16 mars 2013 01 33 36](https://f.cloud.github.com/assets/328718/266120/2eead1cc-8dd1-11e2-95c2-51e703905cbe.png)