added filter option in function_score functions

boost_factor and script_score functions.

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-function-score-query.html#_relation_to_literal_custom_boost_literal_literal_custom_score_literal_and_literal_custom_filters_score_literal
Merge pull request #451 from V1tOr/master

added filter option in function_score functions