User authentication

Add ability for users to log in and log out. Resolves #10 
This is something I'm currently debating. At the moment, I decided to take it out until I can figure out an easy way to style it similarly to my error messages. If I could do `redirect_to root_path, success: 'Article successfully created'`, then it would have the right class so that Bootstrap would style it like I want (via [this](https://github.com/andycandrea/hnews/pull/19/files#diff-9599427925097c3c66f26ac1e0de5cadR42)). Obviously, the look isn't terribly important, so I may just put it back in and write my own basic CSS for it.