Asteroids V0.1.5

Pretty significant visual update to the game. Redid the UFO, asteroid, player, and spikey ball sprite (now a circular saw type enemy). Added background to the main game. Updated the default weapon and the homing projectile. Added credits that will be usable in a future update (hopefully along with the first boss!). Currently 3 weapons are unlocked from the start, I hope on expanding the game and adding some perks, a few more enemy types, and a boss. Additionally I would like to add a shop after every 5 waves or so, where you may buy a perk/weapon with your credits earned.