Implement the `weight` in the function score query

The [weight](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-function-score-query.html#_weight) now replace the `boost_factor` since the 1.4.0.

I also fixed https://github.com/ruflin/Elastica/issues/492 by replacing the `scale_weight` parameter by the `weight`
Please keep the naming of protected variables as they are (naming convention).