Add attribute rel="prev/next" for SEO

Since september 2011, the usage of elements rel=”next” and rel=”prev” is recommended by Google for pagination crawling.
http://googlewebmastercentral.blogspot.fr/2011/09/pagination-with-relnext-and-relprev.html

It is a WHATWG specification too: http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#sequential-link-types