Added fuel and bomb bonuses to airstrike

[Eero Tamminen]

Hi,

I've added/changed following features to Airstrike:

* Bomb bonuses.  Bonus machine can set in level settings to spit out these
  (among other bonuses).  If plane catches it, it will get more bombs

* Fuel bonuses. If plane catches it, it will get more fuel

* Plane has fuel. It's consumed when plane is accelerated.  If there's no
  fuel, plane cannot be accelerated (says "no fuel" instead)

* Separate fuel sprite in addition to fuel bonus. From the plane's point of
  view this works like fuel bonus (it send bonus message on collide event).
  This will be especially handy with generators (+ trigger with e.g. 15 sec
   timeout)...

* Level 3 has fuel sprites, level 4 fuel bonus

* I also resurrected the plane feature where will it will output black smoke
  after damage is 10 below plane hitpoints


	- Eero