Add bootstrap layout

**As a** publisher 
**In order to** ensure my content is readable in different devices 
**I want** the app to be responsive 

**As a** developer 
**In order to** get easy to use responsiveness builing tools 
**And** to be able to customize them easily 
**I want** to add Bootstrap 
**And** I want it to be server through a [Rails-like assets pipeline][assets-pipeline] 


**Note**: the [Jekyll Assets][jekyll-assets] pipeline is really great!

  [assets-pipeline]: http://guides.rubyonrails.org/asset_pipeline.html
  [jekyll-assets]: https://github.com/ixti/jekyll-assets