Modifiers now renew their duration if picked up again
powerup, powerdown and hostile spawning are now correctly adjusted to
game difficulty
Merge branch 'master' of https://github.com/drauthev/space_battle.git
Merge branch 'master' of https://github.com/drauthev/space_battle.git
shield now gets destroyed in case of collision with hostiles
Merge branch 'master' of https://github.com/drauthev/space_battle.git