Move more_like_this query to Tire::Search::MoreLikeThis::Query
Add Tire::Search::MoreLikeThis::Search

* Inherits from Tire::Search::Search
* #initialize is a copy of original method
* Add document_id as attribute
* Change path to /_mlt
Add more_like_this to Tire::Model::Search::InstanceMethods

* Copy of Tire::Model::Search::ClassMethods.search
* Handles mlt_fields
* Searches with Tire::Search::MoreLikeThis::Search