Allow screens to choose their status bar style.

Introduces a new class method called `status_bar`

You can do things like:

```ruby
class MyScreen < PM::Screen
  title "Whatever"
  status_bar :light
end

class MyScreen < PM::Screen
  title "Whatever"
  status_bar :none
end

class MyScreen < PM::Screen
  title "Whatever"
  status_bar :default, UIStatusBarAnimationFade
end
```

The code will automatically handle showing and hiding the status bar for you and setting the style to light or dark.

...for those times you want to manually specify the style.

It will also warn you if you try and use this method without putting ``app.info_plist["UIViewControllerBasedStatusBarAppearance"] = false` in your Rakefile.