docs: def prepare(self, hash, start, end, name, sources, sample=None):
          """ Prepare a historics query which can later be started.
  
              Uses API documented at http://dev.datasift.com/docs/api/rest-api/endpoints/historicsprepare
  
              :param hash: The hash of a CSDL create the query for
              :type hash: str
              :param start: when to start querying data from - unix timestamp
              :type start: int
              :param end: when the query should end - unix timestamp
              :type end: int
              :param name: the name of the query
              :type name: str
              :param sources: list of sources  e.g. ['facebook','bitly','tumblr']
              :type sources: list
              :param sample: percentage to sample, either 10 or 100
              :type sample: int
              :return: dict of REST API output with headers attached
              :rtype: :class:`~datasift.request.DictResponse`
              :raises: :class:`~datasift.exceptions.HistoricSourcesRequired`, :class:`~datasift.exceptions.DataSiftApiException`, :class:`requests.exceptions.HTTPError`
          """
          if len(sources) == 0:
              raise HistoricSourcesRequired()
          if not isinstance(sources, list):
              sources = [sources]
  
          params = {'hash': hash, 'start': start, 'end': end, 'name': name, 'sources': ','.join(sources)}
          if sample:
              params['sample'] = sample
          return self.request.post('prepare', params)