Ensure Views Rotator attaches only once

Views rotator currently re-runs its behavior every time Drupal.attachBehaviors is run. This means that if some AHAH event or other call to Drupal.attachBehaviors occurs, Views rotator can attach multiple times to the same element leading to all sorts of undesirable behaviors.

This patch uses the class addition/check pattern common to Drupal behaviors to prevent this from happening.