Add double Missile Button
Add sound to Main Menu. Add interface Sound to Entity. change Volume for
sounds in game.
Asteroid do more damage over the time. Asteroid are generated faster
over the time.
Change States logic. Add ScoreBonification. Change Bonification logic
for shield.
Refactor World and actors for Single Game Mode in a new packages. Add
WorldState to store actor's state for a specific Game Mode.
Move ScoreBonification to world.single package, because it's only for
single game mode. amended reboot of ScoreBonification, now the timeBonus
isn't restarted.
Merge pull request #1 from jefree/refactor_actors_world

Refactor: actors and world