[Crystal-core] License change

[Jorrit Tyberghein]

Again we changed the default CC license :-)

We used to have http://creativecommons.org/licenses/by-nc/2.5/ as the
default license. We now changed this to the less restrictive
http://creativecommons.org/licenses/by-nc-sa/2.5/ license.

If you feel this is too much for your own artwork that you contributed
to CC then you are free to changed the licensing of your own
model/texture/sound to the by-nc version again. Just let me know.

Greetings,