Backing off on `Exception` too generic

[sholsapp]

It seems that this code is a little too generic. I wonder if you'd be open to a PR that changes the logic here from catching Exception to HTTPConnectionError (or even better, something that understands HTTP response codes).

Thoughts?

[rca]

Not all uses are to backoff from bad HTTP requests.

What about an optional kwarg specifying the exceptions to catch and others are simply bubbled up as exceptions.

With that in place, an easy extension is creating a HTTPBackoff class

Thoughts on that?