Added related plugin

Solved cant see [relatedDocuement](https://github.com/docpad/twitter-bootstrap.docpad/blob/master/src/layouts/post.html.eco "related") :)

I additionally generates a conflict with docpad.coffee line [#80](https://github.com/docpad/twitter-bootstrap.docpad/blob/master/docpad.coffee#L80) 
always show post's pages with tag 'post' using related plugin, but im solve adding meta:

    ---
    name: post
	---
and

    database.findAllLive({name: $has: ['post']}, [date:-1])

I do not know if this is the best practice / solution or is a conlicto really?

bye