CRM-14193 - Add CiviMail component setting to control Dedupe Mailings op...

...tion. This PR replaces PR 2497 which had fatal syntax error (missing comma) in Mailing.setting.php

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* CRM-14193: Add option to default Civimail "dedupe e-mail" setting to true
  http://issues.civicrm.org/jira/browse/CRM-14193