New assets credit

For half legal licencing reason and half thanking reason here are all the creator of the new assets :

BACKGROUND MUSIC https://opengameart.org/users/maxstack

BOSS MUSIC https://opengameart.org/users/nene

BACKGROUND IMAGE  https://opengameart.org/users/rawdanitsu

FIRST BOSS SHIP https://opengameart.org/users/esbxp

3RD BOSS SHIP http://millionthvector.blogspot.com/

OTHER BOSS SHIP https://opengameart.org/users/skorpio

OTHERs SHIP http://creativecommons.org/publicdomain/zero/1.0/

ALL the assets came from https://opengameart.org, thanks to them.