PHP platform frontend & interpretor configurable in `app.yml`

The idea here is to have a PHP platform that is a lot customisable though the `app.yml` file. There's a difference between frontend and interpretor:
- Frontend can be nginx or apache
- Interpretor can be php (though fpm) or hhvm or even php's night build, phpng, ...

**Tasks:**
- [x] Rewrite the platform to chose between frontend & interpretor in `app.yml` file
- [x] Create an `apache-mod-php` frontend as default to keep backward compatibility
    - [x] Support custom Apache vhost with `vhost_file` option
    - [x] Support of custom Apache modules with `modules` option
- [x] Create an `apache` frontend
    - [x] Support custom Apache vhost with `vhost_file` option
    - [x] Support of custom Apache modules with `modules` option
- [x] Create an `nginx` frontend
    - [x] Support custom nginx vhost with `vhost_file` option
- [x] Add `composer` option
- [x] Create an `fpm54` interpretor
    - [x] Support custom `php.ini` file
- [x] Dispatch environment variables to php-fpm interpretor 

I wrote a [`README.md` file](https://github.com/sroze/basebuilder/blob/optionable-php-platform/php/README.md) in which there're explanations about how to use it from a user point of view.