Refetch instance data from the API.

def refresh(self):
      '''Refetch instance data from the API.
      '''
      # There's no GET endpoint for steps, so get the parent guide and loop
      # through its steps until we find the right one.
      response = requests.get('%s/guides/%s' % (API_BASE_URL, self.guideid))
      attributes = response.json()
      
      for step in attributes['steps']:
         if step['stepid'] == self.stepid:
            self._update(step)
            return
      raise Exception('Step with id %s not found in guide %s.' \
                      % (self.stepid, self.guideid))