Add support for clearing scroll_id

According to the documentation, you can abort the scroll search by deleting the scroll_id. Since the API call is the same with the scrolling and clearing the scroll_id but they only differ in the method of the call, I added an option to the scroll action which determines the method of the API call.

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-search-type.html#clear-scroll