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 .
@andycandrea -- this is kind of what I was getting at when we were sitting together.  I don't think the pattern I showed you works as well in this controller because each action has a different lookup mechanism for the `article`.  In which case, setting an instance variable is fine, just so long as the instance variable isn't referenced in a partial.  I recommend always passing any variables needed as `locals` to view partials.  I'm not sure if you were doing this before -- I'm just throwing it out there.  Sorry for the confusion.