Used the NSCoding protocol in ChecklistItem so that we can implement initWithCoder and encodeWithCoder methods for data persistence in local storage.

Implemented initWithCoder and encodWithCoder in ChecklistItem to save/load the ChecklistItem object's text and checked.

Removed dummy ChecklistItems in CheckListViewController.

Created two methods to look up the directory and target data file path in CheckListViewController.

Created saveChecklistItems and loadChecklistItems to use NSKeyedArchiver and NSKeyedUnArchiver using NSData.