Adds BUKKIT-3821 - Fires vehicle events for saddled pigs

The Issue:
A saddled pig currently does not fire any vehicle events. Since it fulfills the definition of a vehicle and extends vehicle already, it only makes sense that it should fire vehicle events.

Justification for this PR:
It makes sense to fire vehicle events for vehicles.

PR Breakdown:
In this PR I am firing the VehicleEnterEvent when an entity attempts to ride the settled pig. I am also firing the VehicleCreateEvent when a saddle is added to the pig, also firing the VehicleDestroyEvent when the saddle is removed. Finally, I am firing the VehicleUpdateEvent and VehicleMoveEvent in their corresponding locations.

Testing Results and Materials:
Tested with VoxelBorder (https://github.com/TVPT/VoxelBorder), player was successfully ejected from the pig when attempting to cross a border.

JIRA Ticket:
BUKKIT-3821 - https://bukkit.atlassian.net/browse/BUKKIT-3821