Update the step using the blob of json-parsed data directly from the
      API.

def _update(self, data):
      '''Update the step using the blob of json-parsed data directly from the
      API.
      '''
      self.orderby = data['orderby']
      self.revision = data['revisionid']
      self.title = data['title']
      self.lines = [Line(self.guideid, self.stepid, line['lineid'], data=line)
                    for line in data['lines']]
      # TODO: Support video.
      if data['media']['type'] == 'image':
         self.media = []
         for image in data['media']['data']:
            self.media.append(Image(image['id']))