1.add background movement 2.add enemy ship
add SpaceShooter.userprefs file
Merge branch 'master' of https://github.com/SamZhangGame/SpaceShooter
Merge branch 'master' of https://github.com/SamZhangGame/SpaceShooter
Merge branch 'master' of https://github.com/SamZhangGame/SpaceShooter