add support for mlt queries (+offline tests)

Hey, I've implemented mlt queries (aka. more_like_this) so now the client supports the shorthand provided by ElasticSearch. For more information:

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-more-like-this.html
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-mlt-query.html

Feel free to discuss the commit and propose improvements or changes. I'd like to see it integrated in the code base.

Thanks.