Added method fieldKeys() to DAOs. CRM-12464

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* CRM-12464: Search improvements in 4.4
  http://issues.civicrm.org/jira/browse/CRM-12464
Refactored out of CRM_Core_PseudoConstant: ufGroup(), currencySymbols(). CRM-12464

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* CRM-12464: Search improvements in 4.4
  http://issues.civicrm.org/jira/browse/CRM-12464