responsive css implementation

Based on media query to not touch the base css.

Do not hesitate to review and to comment if you want something else !

BTW : i have removed the header <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> in the main.scala.html because it's depreciate and need to be moved to the request header, so if you have a front server (nginx, apache...) you need to configure it, else you need to configure playframework for it (if it not the case by default). But today all majors browsers have this default behavior ;)