adding unit test for MailQueueHandler::getAffectedUsers()
Select the minimum amq_latest_send time to avoid issues with postgres & oracle

Fix #92
subjectparams must not be empty
Extend MailQueueHandler::getAffectedUsers tests
Use quotes around column names
Merge pull request #93 from owncloud/issue/92

adding unit test for MailQueueHandler::getAffectedUsers()