-Edited scene to have 3 zombies and 1 turret, instead of 3 turrets and 99
zombies.
-Tweaked values of player shooter and turret shooter to be weaker and
slower.
-Added 'knockback' and 'damage' values to shooter.
-Added 'Enemy' class, which Zombie inherits from. It contains health and
basic enemy behavior.
-Mouseshooter now inherits from Shooter class.
-Added zombie walking and death animation.
-Modified turrets to attack only if the target is alive.