Add setPostFilter method to Elastica\Query

Hi,

Just added a missing method setPostFilter() that allows to set "post_filter" param to Elastica\Query object.
@see http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/_filtering_queries_and_aggregations.html#_post_filter

Pawel
One detail I forgot: Can you add the pull request Id at the end of the line. I just realized the one before is also missing. If you can add also this one, I don't mind ;-)