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 .
@zporter Actually, question on this. Unless I'm missing something, the times I use the instance variable `@comment` are referencing that instance variable in `ArticlesController`. Adding this as a helper method in `CommentsController` doesn't seem to be working, which I'm guessing is due to the fact that `Article` views aren't looking in `CommentsController` for helpers. Does this still make sense?