Add scan helper

[pquentin]

The search with scan, scroll and clear_scroll primitives are implemented in the client, but a scan helper is still lacking. It would work like http://elasticsearch-py.readthedocs.io/en/master/helpers.html#scan

For Python 3.6+, it could be an asynchronous generator, which would be closest to the official ES client.
For Python 3.5+, it could be an async for:

async for hit in Scan(
        client=client, index=..., doc_type=...,
        query={"query": {"bool": {"filter": ...}}}):
    ...

The only way to support Python 3.4 would be to add a callback, I guess, but that's not a good fit with asyncio's model, so maybe 3.4 can be dropped for this feature.

Thoughts?

[hellysmile]

@pquentin Hey, fill free to copy-paste https://github.com/wikibusiness/aioelasticsearch/blob/master/aioelasticsearch/helpers.py

[pquentin]

Thanks @hellysmile. Closing.