Registers an action to happen on an event. The action can be any sort
of callable:

Events::on('create', 'myFunction');               // procedural function
Events::on('create', ['myClass', 'myMethod']);    // Class::method
Events::on('create', [$myInstance, 'myMethod']);  // Method on an existing instance
Events::on('create', function() {});              // Closure

@param $event_name
@param callable   $callback
@param integer    $priority