Add more like this API call

This is more a proof of concept if and how to implement `_mlt`
http://www.elasticsearch.org/guide/reference/api/more-like-this/

This is how to call it currently
```ruby
tire.more_like_this(page: params[:page], per_page: params[:per_page], mlt_fields: %w(title subtitle abstract series journal), min_term_freq: 1, min_doc_freq: 2) do
  query do
    all
  end

  Rails.logger.debug to_curl
end
```

* I don't like that I had to copy so much from other methods. Would love if I could solve this with even much less code
* There are no tests

I would love to hear feedback on this. Am I doing it right? What would you do differently?