Add automatic computation for exponential backoff in retrier
Add automatic computation for exponential backoff in retrier

Signed-off-by: lsenta <93e2db26c7714cc3fb230d05f2fbedc450bb1db9@gmail.com>

torymur/python-hubstorage,kalessin/python-hubstorage,scrapinghub/python-hubstorage