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 .
is that `created_at` being spit out like `"Tue, 20 May 2014 14:24:26 MDT -06:00"`? [`strftime`](http://apidock.com/ruby/DateTime/strftime) might be useful here