Update 0.9.8f14 - Big graphic improvements

Hi everyone.

Big graphic changes has been applied to the game.

Here's all the changes made with last update:

New features:
• New skybox system with volumetric clouds (HD Sky must be enabled in options)
  
• New achievement: Flying Ace.
  
• Added Stuka G-1 with 37mm cannons.
  
  Changes:
• Improvements in performance and optimizations.
  
• Improvements in lightning.
  
• Changes in tracers.
  
• AI doesn't get stuck anymore.
  
• Fixes in planes.
  
• Various other fixes.
  

Thank you all,
Marco.