Allow assigning god-mode players to vehicles

[NiftyManiac]

A god-mode player with no character can still enter vehicles and travel with them, and the vehicle is visible through player.vehicle, but vehicle.passenger is nil. As far as I can tell, there's no way to assign a god-mode player to a vehicle.

I assume multiple god-mode players can follow a single vehicle, so maybe entities should have a writable "observer" field which lists all players for whom player.vehicle would return that entity. Alternatively, allowing the player.vehicle field to be writable would solve this as well (but might conflict with handling of passengers).

[Rseding91]

Added for 0.15.

I also found and fixed a problem with the logic in entering/exiting vehicles around this as well.

[NiftyManiac]

Sweet, thanks.