Music, SFX, new enemy and random enemy spawning

Collisions: The ship now collides with enemies and looses hp. After the hit, it gets 2sec immunity.

Music and SFX: Background music and shoot/hit/destroy/game-over SFX.

New enemy: A "probe" enemy that will shoot at you!

Random enemy spawning: The enemies now spawn randomly. No incremental difficulty for now, but it's good to know that I got rid of level design... ;)

New ship sprite: