Add "notes" to items

[jsealey]

Changes to be made:

to AddItemViewController AND EditItemViewController:

• add the "notes" text box to the view in the storyboard and link to the code
• when the text box becomes first responder, shift the window down so users can see the whole text box.
• when the text box resigned first responder, shift the window back up to its original position

to model:

• add "notes" to the "add item" and "update item" functions.

*Notes is already a field included in the xcdatamodel for the Item entity. I already added it (along with category) to the Item class so we wouldn't have to migrate the database in later updates.