Allow for more request params (see https://github.com/ruflin/Elastica/pull/291)

Apparently my changes form https://github.com/ruflin/Elastica/pull/291 got lost in your subsequent refactorization, so here I am again :-)

For the record, that allows for all parameters listed in  http://www.elasticsearch.org/guide/reference/api/search/request-body.html.

Thanks!