Use function_score for complete queries

I implemented function_score scoring ( see http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-function-score-query.html ) on the top level of the query.

It's a quite basic implementation, but it should do for our production needs. Specs are there, so please have a look and let us know if you can merge this.
There is no body `:filter` key currently, it was redone properly as `:post_filter` and filter is included in body here.