Added a JB/related_posts helper which is WAY more accurate than site.rel...

Added a JB/related_posts helper which is WAY more accurate than site.related_posts that Jekyll is bundled with. And here's why :

The method does like this :

- Fetches all the site.posts
- Compare their categories with the current post's categories
	- If they (site.posts.post and page) got the exact set of categories -> display the link to the post
	- If they got at least one category in common -> display the link to the post too
- All the mechanism is limited by site.JB.related_posts_max so you can set a site-web maximum for this helper