Allow a jitter value to be passed in

To avoid the thundering herd problem when retrying is
triggered at exactly the same moment by many entities
allow for passing in a jitter maximum value which will
be used to randomly alter the amount of time slept
by each retrying object user; this helps randomize the
thundering herd and makes it possible to reduce the
issue.
Merge pull request #23 from harlowja/jitter

Allow a jitter value to be passed in