Starting to clean errors

* Clean some errors (some left in `install.php`)
* Add CC BY SA infos in `README`
Merge branch 'master' of https://github.com/Phyks/Freeder
Refactor installation according to
https://github.com/FreederTeam/Freeder/commit/664462c8cc9d5d381b872a4b22f2680f0b7b3d09#commitcomment-7165979