Added support for Core Data
Core Data will be used to store to-do items

- For future reference, select "Use Core Data" when creating new project
- To add it manually, I
  a) Copied the default code for Core Data into the app delegate .h and
     .m files
  b) Created a new file with the Data Model template
  c) Added the CoreData framework
  d) Added a CoreData.h import directive to the .pch file
Added a .gitignore for the persistent .xcuserstate file