Cleanup: Use Phaser.Bullet & Phaser.Weapon

Cleanup: Use Phaser.Bullet & Phaser.Weapon

I realized I reinvented the wheel by implementing my custom projectile
sprite derived from Phaser.Sprite. There are predefined classes in
Phaser.js just for this purpose. Now the player's weapon is derived
from Phaser classes.

	modified:   ld40.js