'Add a new item to the book manifest.
The item\'s :param:`id`, :param:`href`, and :param:`media_type` are all
required.  A :param:`fallback` item-id is required for any items with a
MIME type which is not one of the OPS core media types.  Either the
item\'s data itself may be provided with :param:`data`, or a loader
function for the data may be provided with :param:`loader`, or the
item\'s data may later be set manually via the :attr:`data` attribute.'

def add(self, id, href, media_type, fallback=None, loader=None, data=None):

    item = self.Item(self.oeb, id, href, media_type, fallback, loader, data)
    self.items.add(item)
    self.ids[item.id] = item
    self.hrefs[item.href] = item
    return item
