Associate users and articles; require login to post

Resolves #12.
I'd have better feels if you weren't directly accessing the `session[:user_id]` here.  Since you've already handled session management with your `SessionsController` and created helpers in your `ApplicationController`, I would use those helper methods rather than injecting knowledge about the session and its contents into additional controllers.
```
@article.user_id = current_user.id
```