CRM-14486 add empowered by CiviCRM as public footer

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* CRM-14486: Empowered by CiviCRM - footer added to all public pages (with option to disable)
  https://issues.civicrm.org/jira/browse/CRM-14486
Merge pull request #3246 from agh1/empowered-by

CRM-14486 add empowered by CiviCRM as public footer