Drop useless license statement about the photograph of the NGC3606 nebula

The concerned file is textures/back01.png which is a modified version of
http://hubblesite.org/newscenter/archive/releases/1999/20/image/a/

This photograph is licensed in the public-domain by the NASA, which is
itself compatible with the GPL-3.
Replace CREATIVE-licensed thunder sound by a public-domain thunder sound

This thunder sound was downloaded from
https://archive.org/details/TheSoundOfThunder and was converted to WAV
using audacity.
Merge pull request #7 from OdyX/dev-fix-licensing-problems

Fix licensing problems