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.