Scroll support

[nickkaltner]

Initial implementation of a scroll function. I just wanted to get some feedback before i firm it up. I think it's good, it's kinda nice to be able to handle the search as a stream. But the downside is you won't get aggregations.

[tomtaylor]

Thanks for this, it's a nice improvement. Let me have a think about the API for it, but I think we should definitely include something like this. I'll follow up when I get a moment.