Added header view to the items list.
Add Item Cell for items list.
Merge remote-tracking branch 'origin/bogdan' into sprint2_mircea
Refactored add item views and others.
Category picker changes and small refactoring in add item functionality.
Merge branch 'bogdan' into sprint2
Delete CategoryItems class.
Add CategoryItem and ShoppingListItem tables.
Add ShoppingListCategory class.
Add plist files for CategoryItem and ShoppingListItem.
Add protocol for Model class.