Added Celery task for queued email sending.

Upon IOError or OSError (which includes SMTP protocol errors) the mail
sending task is retried after MAIL_RETRY seconds. It is retried three
times (default setting of Celery) only.

pillar/__init__.py

@@ -462,6 +462,7 @@ class PillarServer(BlinkerCompatibleEve):
             'pillar.celery.tasks',
             'pillar.celery.algolia_tasks',
             'pillar.celery.file_link_tasks',
+            'pillar.celery.email_tasks',
         ]
 
         # Allow Pillar extensions from defining their own Celery tasks.