Vehicle cleanup

Taking this over from @edisonout to clean it up a little before the Yahoo! milestone is closed out and get some of the issues mentioned in #2125 addressed.

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Dismounting is now done using the same key as mounting to eliminate confusion. Dismounting near a door will not go through the door so you do not accidentally leave the level.

Movement animations for vehicles are dependent on player movement velocity.

Not sure if I'll get to attacking, but flying may be possible in the next couple of days.