Support modal transition and presentation styles.

You can now do things like this:

```ruby
    open_modal DetailScreen.new,
      nav_bar: true,
      transition_style: UIModalTransitionStyleFlipHorizontal,
      presentation_style: UIModalPresentationFormSheet
```

I'll update the wiki once the branch is merged... not sure how I can add tests for this one, but all current tests are passing.