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

Either `**kwargs`, or `raw_results` is needed. I doubted and opted for the former, or would you rather have arguments explicit when they are only used in the parent's class?