This is a generic test for sotrages implementing the
``IEditorialRating`` inteface, which stores a single rating for each
object.  Ratings are retrieved as IRating objects.  New ratings on the
object overwrite the old rating::

  >>> from datetime import datetime
  >>> rating = storage()
  >>> print rating.rating
  None
  >>> rating.rating = 5
  >>> float(rating.rating)
  5.0
  >>> rating.rating = 10
  >>> float(rating.rating)
  10.0
  >>> isinstance(rating.rating.timestamp, datetime)
  True
  >>> print rating.rating.userid
  None
