CRM-12683 - Load extensions in "vendor" directory

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* CRM-12683: Load extensions in "vendor" directory
  http://issues.civicrm.org/jira/browse/CRM-12683
CRM-12683 - Fix NPE when an extension container disappears

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* CRM-12683: Load extensions in "vendor" directory
  http://issues.civicrm.org/jira/browse/CRM-12683
Merge pull request #837 from totten/vendors-extdir

CRM-12683 - Load extensions in "vendor" directory