Convert from .xib files to storyboard. Now the storyboard segue presents addNodeViewController.

TODO: In TrinaryTreeViewController.m eliminate unused methods to present addNodeViewController.
Finish convert from xib files to storyboard. Delete AddNodeViewController.xib

In TrinaryTreeViewController delete instantiate addButton, it's done in storyboard.
Change segue from modal to push.

In AddNodeViewController delete protocol AddNodeViewControllerDelegate and delegate property.
Use storyboard to instantiage navigation buttons, connect them to IBActions.
In dismissView: use popViewControllerAnimated: