CRM-14159 API Add generic options['metadata'] = labels functionality

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* CRM-14159: Add ability to return labels as top level metadata
  http://issues.civicrm.org/jira/browse/CRM-14159
code/comment cleanup
test cleanup
Merge pull request #2454 from eileenmcnaughton/CRM-14159

CRM-14159 API Add generic options['metadata'] = labels functionality