added sound effects; explosion & laser shot
Merge branch 'master' of https://github.com/fpruitt/negaGalaga

# By Forrest Pruitt
# Via Forrest Pruitt
* 'master' of https://github.com/fpruitt/negaGalaga:
  Fixing Camera in MainGameScene
  Enemy Placed in MainGameScene
  Re-cut sprite sheet to be flipped.