Add a random exponential wait strategy; Fixes #28

[NJAldwin]

This adds a variant of the exponential wait strategy that waits for a random time between the minimum provided time and an exponentially increasing maximum time (under a maximum provided time). This is similar to https://en.wikipedia.org/wiki/Exponential_backoff

See #28

[NJAldwin]

@rholder any chance you'll be able to take a look at this sometime relatively soon? (no rush, just wondering if I should just implement this in our code for now or wait for this to be merged)

[rholder]

This looks pretty good, and it's first on my TODO list of maintenance when I circle back to this project. I should have bandwidth to ship something this week. Keep pinging this issue as a reminder if you don't see some movement soon.

[skwslide]

Any chance of committing and releasing this feature soon? Thanks.