Support user-account migrations

Thanks, it looks good for the most part. A couple of improvements need to be made though.

The complete() and prepareRollback() should use code from the common class. Here's an example from node.inc.

  /**
   * @param $entity_id
   */
  public function prepareRollback($entity_id) {
    $common = new MigrateDefaultContentCommon();
    $common->rollbackEntity($entity_id, 'node');
 }

  /**
   * @param $entity
   * @param stdClass $source_row
   */
  public function complete($entity, stdClass $source_row)  {
    $common = new MigrateDefaultContentCommon();
    $common->completeEntity($entity, 'node');
  }

The 'addReference' and 'resolveReference' methods are now handled by the common class as well and should be removed.