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 was done as a result of your comment [here](https://github.com/andycandrea/hnews/pull/24#discussion_r14792473). The comment instance variable had been used in one of my `Article` views, so I wrote the `comment` method as a helper so that it could be called instead.