Add the ability to push a Screen on a navigation_controller without animation

Hi,

I've added the ability to push a screen on a navigation_controller without animation.

Example:
```
open_screen(MyScreen, { animated: false }
```

Tests are ok, and here is the result for my changes.
```
bundle exec rake spec files=spec/unit/screen_helpers_spec.rb
34 specifications (62 requirements), 0 failures, 0 errors
```