Support iOS 6 force rotation.

So i finally figured out my problem with the portrait orientation stuff.

I've added a few methods to the `NavigationController` subclass to support auto-rotation based on supported interface orientations.

Essentially, all you have to do in your view controller now is this:

```ruby
  def shouldAutorotate
    true
  end

  def supportedInterfaceOrientations
    UIInterfaceOrientationMaskPortrait # Or any other orientation mask
  end
```

With this patch and that code in the VC, that VC will *ALWAYS* present in portrait mode.

**Note: this hasn't been tested for iOS 5 - my 5.1 simulator crashing on load (unrelated issue to this code)**