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 .
When I check that the posts are ordered properly on the page, I do 'page.body.index(new_article.title).should < page.body.index(old_article.title)'. I changed it from `'Title 2'` to `'Second title'` due to the fact that some of my `have_content` lines to `have_link` to be more specific, and the `'2'` in `'Title 2'` was confusing the test that was checking for the link to the second page, which is `have_link('2')`.