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 .
That's correct, but it'll be one less thing to think about if the deletion of users is ever allowed.  The [`delegate`](http://api.rubyonrails.org/classes/Module.html#method-i-delegate) method could also be used on the `Article` class which has an `allow_nil` option, but it's probably preferable not to show this `<div>` at all if there isn't an associated user.