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.

----------------------------------------
* CRM-8338: Mailing Label Household Merge requires Household to be selected as well as individuals
  http://issues.civicrm.org/jira/browse/CRM-8338
CRM-8338 removed CRM_Contact_Form_Task_Label::mergeSameHousehold(), made obsolete by CRM_Contact_Form_Task::mergeContactIdsByHousehold() implemented in previous commit.

----------------------------------------
* CRM-8338: Mailing Label Household Merge requires Household to be selected as well as individuals
  http://issues.civicrm.org/jira/browse/CRM-8338
Merge pull request #1773 from davejenx/CRM-8338-merge-by-household

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