GroupContact enhancement to add contacts to groups with Pending status

Currently, the API only allows contacts to be Added or Removed to a Group. CiviCRM also supports the Pending status, but this is not available in the API, and so can't be utilised without burrowing deep into CiviCRM itself.

I attach a series of small enhancements to api/v2/GroupContact.php to support the Pending status for group contacts.