addGroceryListItem() and removeGroceryListItem()

Added helper functions for adding and removing grocery list items
to/from the datastore.

The addGroceryListItem() function will create the new item in the
/objects tree, then update the 'objects' index for the parent container.

The removeGroceryListItem() function will remove the item from the
parent container's 'objects' index and then remove the item.