Created toDoItem object; dynamic prototype table cells

- New class to define a to-do item (one cell in the to-do list).
- Changed to-do list cells from static to dynamic prototype to
    allow cell content to be updated via the data source in the
    view controller.
Items can be marked as completed

- Added tableView:didSelectRowAtIndexPath to toggle item completion
- Completed items have a checkmark (cellForRowAtIndexPath)
Can now add new items to to-do list

- Add referencing outlets for Done button and New to-do item
    text field in AddToDoItemViewController object
- If Done button selected, create new toDoItem
- In TDLToDoListViewController, update table with new toDoItem
    via the unwind segue
Last commit from Start iOS tutorial

- I have finished the tutorial
- Any commits after this one will be done on my own