Auto-Layout constraint: Tob of NavBar to Top Layout Guide
Make NavBarDelegate's positionForBar return UIBarPosition.TopAttached:

  UIBarPositionTopAttached

  Specifies that the bar is at the top of the screen, as well as its containing view.

  Bars with this position draw their background extended upwards, allowing their
  background content to show through the status bar.
Merge pull request #1 from dhoepfl/master

Auto-Layout constraint: Tob of NavBar to Top Layout Guide