CRM-13690 - CiviMail - prevent html2text treating mailing tokens as relative URLs

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* CRM-13690: html2text fails to convert Mailing token links
  http://issues.civicrm.org/jira/browse/CRM-13690
Merge pull request #2013 from kenwest/CRM-13690-B

CRM-13690 - CiviMail - prevent html2text treating mailing tokens as rela...