Bazooka Works + Added Boss + shield functionality

Bazooka now kills every enemy and lowers health of player by 1.
Added a boss Ufo that will spawn every score multiplier of 15 ie
15/30/45 etc
SHIELD BUFF and NERF - BUFF : Player can now shoot why using shield
NERF - Shield only has 2 life points. When enemies can eventually shoot,
the shield life points will not be decreased when blocking enemy
projectiles