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 .
I actually didn't have this originally, but I saw @pk-nb did it. From looking [here](http://guides.rubyonrails.org/layouts_and_rendering.html#using-partials) (look right above 3.4.5), it seems to be taking advantage of the following:

> <%= render @customer %>
> Assuming that the @customer instance variable contains an instance of the Customer model, this will use _customer.html.erb to render it and will pass the local variable customer into the partial which will refer to the @customer instance variable in the parent view.