Added Background Music

Hey its me again, all I did in this update was add some background music to the game.

I might end up adding some sound effects to, for the shooting, destroying enemies and the player death.

Known Bugs:

• When you loose the game and click on the start game button two times the amount of enemies will spawn, if you do it again, three times the enemies will spawn, and so on. For now i just recommend, if you've lost the game and want to play again, refresh the page and then play.