Introduce a little randomness into "retry_delay"

Add the ability to specify either "retry_delay_multiplicand_min" or
"retry_delay_multiplicand_max" or both. These values will be used by
the "ExponentialBackoff" retry-strategy. By tuning these values you
can stagger retry attempts in situations where a large number of jobs
fail around the same time (e.g. rate-limiting/throttling or connectivity
issues).
Merge pull request #95 from jzaleski/jwz-add-randomness-to-retry-delay

Introduce a little randomness into "retry_delay"
Update README.