Added ground collision detection.

Added ground collision detection.
Did some serious horsing around to be able to "hug" the ground properly
  and still be able to take off.
Set the near clip plane to 1.0 meters when less than 10 meters above the
  ground.
Did some serious horsing around getting the initial airplane position to be
  correct based on rendered terrain elevation.
Added a little cheat/hack that will prevent the view position from ever
  dropping below the terrain, even when the flight model doesn't quite
  put you as high as you'd like.