Server: PHP coding conventions

This is a proposition to use `php-cs-fixer` <https://github.com/fabpot/PHP-CS-Fixer> in this project to make the code respect PSR-1 and PSR-2 PHP coding conventions.

This commit comes with a makefile to run the `fix` task, together with a dependency on `php-cs-fixer` managed with Composer.

I documented the conventions, makefile and tasks in the readme.
À quoi sert `|| true`, dans ce cas de figure ?