Add a core data model for puzzles

Integrate CoreData with the app, and create a simple
model for storing puzzles.
Add save button to puzzle editor

Add a save button to the puzzle editor to allow
users to save a puzzle to Core Data
Make EditorWindowController a view controller

Replace EditorWindowController as the main window
controller with PEMainWindowController and PEMainWindow.xib.
Move the editor code into an NSViewController.
Add a Puzzle selection menu and save button

Add a save button to the kakuro editor, and launch
the app into a menu from which you can select
a saved puzzle.