Commenting

Add commenting feature to Hacker News clone. Also formalizes the relationship between `Articles` and `Users` and includes stylistic changes and some basic refactoring. Resolves #12 .
This could be written as `<%= article.user.try(:name) || 'Anonymous User' %>`.  Check out the docs for [`try`](http://api.rubyonrails.org/classes/Object.html#method-i-try)