Initial support for CoreData

- just setup work; no saving is being done
Migration to Core Data: part 1

- The lists are now being saved to Core Data instead of NSUserDefaults.
But the actual ToDo items are not being saved yet (part 2 work)
- Changed all ‘NULLS’ to ‘nil’
- Added Data model for Core Data
Changed to-do image type to NSData

- Convert to NSData so we can save to Core Data and also takes up less
space
Migration to Core Data: part 2

- The to-do items now save to Core Data instead of NSUserDefaults
- Code cleanup
Changed 'completed' column type to Boolean
Merge pull request #15 from akshay23/akshay/feature

Lists and to-do items are now saved via Core Data