Chronological links plugin - re #466

New chronological links plugin as discussed on issue #466.  

When the plugin is included in settings adds four variables to the article metadata:
- next_url
- next_title
- previous_url
- previous_title

It can then be used in a template like this:

    {% if article.previous_url %}
    <a href="{{article.previous_url}}">&lt; {{article.previous_title}}</a>
    {% endif %}