Don't pass kwargs everywhere, but do keep params to Elasticsearch.search separated

A combination of #91 and #101. As suggested in https://github.com/openannotation/annotator-store/pull/101#issuecomment-57284831

There was actually no need for `kwargs` here anyway. And I'm afraid Python won't change this imperfection in its syntax anytime.. Perhaps I need to fork that project.. ;)