Fire booking_added event when a slot is created with an appointment

When a slot is created and you book a student onto that slot as a tutor, it does not fire the booking_added event. 

We would like to use that event to send an email to a student to inform them that they have been booked onto a session by the tutor. Currently we rely on the tutor to tell the student that trisection has occurred, but it would be great if this could be automated, as the plugin already does a great job of sending emails and firing events when slots are booked.

We are happy to create a PR for this work if that helps the process. 

Kind regards,

Kevin.