- Add API to get vehicle creation reason, and make VehicleCreateEvent cancellable. Adds Bukkit-3074

Firstly, this includes a reason for a Vehicle's creation, but the heart of this PR is in its implementation.

It was previously impossible to cancel this event because of how this was handled on the server. I made the appropriate changes in the server code, so this needed some additional changes.

CraftBukkit commit here: https://github.com/Bukkit/CraftBukkit/pull/942