Closing </ul> tag in defaut.html

Love this theme, just noticed an unclosed `<ul>` tag.

Also would you be open to a second PR that moves the [navigation in `default.html`](https://github.com/elisehein/Pageturner/blob/master/source/_layouts/default.html#L39) to a partial in the `includes` directory (so it matches [Octopress' instructions to customizing navigation](http://octopress.org/docs/theme/template/#changing_navigation))?