Make sure Article::introduction is parsed with Markdown

Just trigger a call to the markdown helper in the articles' decorator
to make sure we can use the Markdown syntax in any article's intro
Reorganize the stylesheets architecture

The previous stylesheets organization was a bit a mess and some CSS
elements weren't stored in the right place. This commit improve the
stylesheets folder architecture with the following sub-folders:

  * components : ui elements (e.g. code span, tables, etc.)
  * layout : sheets related to the global layout
  * modules : stored the resources related files
  * settings : partials for variables, mixins, etc.

All this files are then loaded into the main application's stylesheet
The site looks the same (with very few fixes I've made on the way)
Wrap the code into a div when no lexer is found

Ensure code is wrapped into a highlight div when the passed lexer
isn't found by Pygments.rb (using fenced code blocks)