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 .
Couple lines in [here](https://github.com/andycandrea/hnews/blob/c18583f56a1d08b368ce02fc818ac1beee416ab0/app/views/articles/index.html.erb#L3) and [here](https://github.com/andycandrea/hnews/blob/c18583f56a1d08b368ce02fc818ac1beee416ab0/app/views/articles/show.html.erb#L1).