Add Previous and next links for articles

[__This diff will be easier to follow, as this branch is based off of #845 and not master.__](https://github.com/moneyadviceservice/frontend/compare/link_recirculation...previous_next_article) Probably merge #845 first.

Looks like this:

![screenshot 2014-12-23 15 36 34](https://cloud.githubusercontent.com/assets/1007202/5539446/897bf850-8ab9-11e4-8832-116f94bac05e.png)

Code looks roughly as you'd expect, adding methods to `Core::Article` called `next_link`/`previous_link`.

See moneyadviceservice/cms#120
I think you might be able to drop this if the next line is:

``` ruby
Array(related_content['popular_links']).map do |popular_link|
  #...
end
```