Add a tag to an individal item.

        tag string must be in form "user/-/label/[tag]"

def addItemTag(self, item, tag):
        """
        Add a tag to an individal item.

        tag string must be in form "user/-/label/[tag]"
        """
        if self.inItemTagTransaction:
            # XXX: what if item's parent is not a feed?
            if not tag in self.addTagBacklog:
                self.addTagBacklog[tag] = []                
            self.addTagBacklog[tag].append({'i': item.id, 's': item.parent.id})
            return "OK"
        else:
            return self._modifyItemTag(item.id, 'a', tag)