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 .
What about replacing the use of the `@comment` instance variable in the views with `comment` and remove this line?  I've just run into problems with using instance variables in views in the past because if they aren't set, then they come up as `nil` which makes it a bit more difficult to debug.  Using a helper method like this in the view will at least throw an `UnknownMethodError`, which is a bit easier to debug and fix.