Catch and retry wait_for_task() methods on HTTPError exceptions.

The requests library brings in its own version of `urllib3`, so we have
to catch `requests.packages.urlib3.exceptions.HTTPError` to make sure we
catch all urllib exceptions.
Formatting
Merge pull request #29 from Nextdoor/fix_issue_28

Catch and retry wait_for_task() methods on ProtocolError exceptions.