add support for mlt queries (+offline tests)
fix mlt query to handle multi-value attrs

While writing online tests, it became clear that `fields` should have
been `mlt_fields` and that `search_indices` and `search_types` were
missing. According to:

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-more-like-this.html#search-more-like-this
add online tests for mlt() (+plus setup data)
Merge pull request #25 from saalaa/master

add support for mlt queries (+offline tests)