Return the stats history (JSON format).

def get_stats_history(self, item=None, nb=0):
        """Return the stats history (JSON format)."""
        s = self.get_json_history(nb=nb)

        if item is None:
            return self._json_dumps(s)

        if isinstance(s, dict):
            try:
                return self._json_dumps({item: s[item]})
            except KeyError as e:
                logger.error("Cannot get item history {} ({})".format(item, e))
                return None
        elif isinstance(s, list):
            try:
                # Source:
                # http://stackoverflow.com/questions/4573875/python-get-index-of-dictionary-item-in-list
                return self._json_dumps({item: map(itemgetter(item), s)})
            except (KeyError, ValueError) as e:
                logger.error("Cannot get item history {} ({})".format(item, e))
                return None
        else:
            return None