Merge branch 'feature/git-attributes' into 'stable-3.1'

Git attributes file to make sure Windows developers don't commit bad line endings.

To obey http://git.ispconfig.org/ispconfig/ispconfig3/blob/45e9406a0fd6e872219a09d913bdfa49bd22e87b/CODING_NOTES.php.txt#L3

https://help.github.com/articles/dealing-with-line-endings/

See merge request !290