Article styleguide

Basic style guide to show base styles, components, layout and an article page with all the elements we are currently using in the alpha.
Refactored govuk_static css into components and base css
Added sass linting


We need it otherwise the layout of the application is being used due to inheritance.

See: [Rails Guides - Layouts and Rendering - Layout Inheritance](http://guides.rubyonrails.org/layouts_and_rendering.html#layout-inheritance)