Added ability to add reminder for an item

- User can now create a reminder for to-do item
- Phase 1 work (will need to clean up)
Fixed bug in AddToDoItemVC

We were not instantiating new ToDoItem object when adding new item.
Instead we were re-using existing object.
User can choose pic from Photo Library

- Added ability to choose pic from library in addition to camera
- Added some comments
Added unique id to ListItem object

- Unique id is used to add objects to a Dictionary so that we can
re-use view controllers instead of creating one everything for a list
Bug fix for deleted to-do items

- The fix deletes/removes reminders for to-do items that are being
deleted
- And some code cleanup
Merge pull request #12 from akshay23/akshay/feature

User can add reminder to to-do item