Fix something bugs:
 - Shield restart when restart game
 - Ship cant shot when game over
 - Reduce the ship rotation speed
Move Message version from World to GameUI.
add centeredX and centeredY methods , refactor centered method.
Add new methods to ScreenUtil
add new Font
Add score variable to world. When a Asteroid is destroy using a Missile
score is update.
Add Label for World.score.
Change font.fnt for Comic Sans MS font
Fix position of Message Label