Book added to a course - send email

[Daníel Sigurgeirsson]

When a teacher registers a book in a course, an email should be sent automatically to the agency which orders books, informing about the change. This should use the MessageQueue system, i.e. the API should post the event to the MessageQueue, and a separate listener should handle this (perhaps using our yet-to-be-written IfThisThenThat clone?)