CRM-8338 implement CRM_Contact_Form_Task::mergeContactIdsByHousehold(), ...

...replacing ids of household members with the id of their household so we can merge labels by household without needing the households to be in the search results.

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* CRM-8338: Mailing Label Household Merge requires Household to be selected as well as individuals
  http://issues.civicrm.org/jira/browse/CRM-8338