Adds a new field `extract` to the articles table

 new file:   db/migrate/20140219102027_add_extract_to_articles.rb
 modified:   db/schema.rb
 modified:   spec/models/article_spec.rb
Creates a method to return an extract from an Article's content

 modified:   app/models/article.rb
 modified:   spec/models/article_spec.rb
Ensures that when an `extract` is created from `content`, HTML is removed

 modified:   app/models/article.rb
 modified:   spec/models/article_spec.rb
Ensures that `extract` does not contain HTML when saved

 modified:   app/models/article.rb
 modified:   spec/models/article_spec.rb
Creates a layout that can add meta tags to the <head> element

 new file:   app/views/layouts/social_media_enabled.html.erb

 modified:   app/controllers/articles_controller.rb - only use layout on show post/page actions
 modified:   app/views/layouts/application.html.erb
Adds a basic set of OpenGraph meta tags

 modified:   app/views/layouts/application.html.erb - allows attributes to be passed to <html> element
 modified:   app/views/layouts/social_media_enabled.html.erb - set OpenGraph namespace and basic meta tags
Adds Facebook namespace to <html> element and fb:admins meta tags

 modified:   app/views/layouts/social_media_enabled.html.erb
Adds twitter cards to social media layout

 modified:   app/views/layouts/social_media_enabled.html.erb
Merge branch 'social-meta-tags' into develop-blog

 Adds new field to articles table for an article extract
 Creates method to create an extract from content if none given
 Creates custom layout, which adds social media meta tags
 ArticlesController#show and #show_page actions can use this layout
Formats the published_at datetime in the HTML5 <time> element

 modified:   app/views/articles/_article_header.html.erb
Adds meta viewport tag for responsive design

 modified:   app/views/layouts/application.html.erb