mod_turnitintool Trigger an event on assignment submission

This allows developers to hook in and perform actions when a TII
submissions is submitted.

This event follows the same naming scheme as discussed in MDL-35710.

Sorry - just remembered that I'd not included the events definition and that Moodle 1.9 doesn't support events. This should only attempt to trigger the events on supporting systems (e.g. Moodle 2).