CRM-11958 - Add hook_civicrm_alterMailer (with unit-test)

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* CRM-11958: New hook implementation for altering outgoing mail configuration
  http://issues.civicrm.org/jira/browse/CRM-11958
Merge pull request #1599 from totten/master-mailer-hook

CRM-11958 - Add hook_civicrm_alterMailer (with unit-test)