SnowMobile, SnowTrail changes

SnowMobile now has good air animation, and properly collides with
objects in the air. It also will try to jump over logs.
New field for all objects: public boolean air, which says whether the
object is in the air or not. This helps with the collision detection.
SnowTrails now fade out after 100 frames, helping performance hopefully.
SnowMobiles have their own SnowTrails, which are spawned once every
three frames and also fade out, this majorly helps performance.
I also nerfed the SnowMobiles. They now spawn with getRandomNumber(2000)
instead of 1000. Also, they don't spawn at the boarder's exact
x-coordinate anymore. Let me know if you think this is too much of a
nerf.