[PoC] Introduce StackPHP & middlewares usage to improve code

I was playing around with [StackPHP](http://stackphp.com/) last days, and I decided to give a try to handle problems we have or we can have in near future while Sylius grows.

This first __PoC__ comes with __very simple__ and __requires__ polishing, as well as changes in other places of code (settings, locales).

This comes as one of strategies that can be extended & be an _alternative_ for #956.

> __Note__:
> Please don't comment that something is not working, it's not production ready, nor finished approach (yet). For now please consider this as idea for extending & possible way of improvements.