Synchronise with existing organic groups when enabling civicrm_og_sync.module

This patch implements the drupal hook_enable() api for civicrm_og_sync.module. Whenever the module is enabled it will scan all existing Organic Groups on the site and ensure that CiviCRM groups and group memberships are appropriately updated.

Without this patch you cannot use civicrm_og_sync.module on a site with existing OG groups due to constant fatal errors.