CRM-15428 - CiviEvent - Add participants to a group

Adds the ability to add participants records to a group via the actions dropdown on a participants list page.
https://issues.civicrm.org/jira/browse/CRM-15428
Merge pull request #4415 from rnao/add-participants-to-group

CRM-15428 - CiviEvent - Add participants to a group