added a unique constraint on list_id, item_id, to prevent adding the same item twice to the same list
changed the /item/create route to /item/add_to_list, since that's really what it is.  I also added a can_add_item() method to Grocery::List, to check the (list_id, item_id) unique constraint before trying to add it.
added .gitignore
removed the database itself from version control
oops, I forgot to do a speculative load on the item, which may or may not be new