scan and scroll object for distributed search execution

Hi,
i wrote a new class for for the [scan and scroll](http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/scan-scroll.html) feature. You can loop over huge amounts of data with a simple foreach.

I never contributed to Elastica and i'm not sure, if you want complex operations like this in your library. But i decided to make a pull request to discuss it :D