Add retries to all HS requests

[shaneaevans]

Not only batch writes as we currently do.

In particular, retry these requests:
http://support.scrapinghub.com/topic/369068-network-timeouts-to-storagescrapinghubcom/

Probably using request HTTPAdapter retries:
http://www.python-requests.org/en/latest/api/#requests.adapters.HTTPAdapter