txretry provides a Twisted class, RetryingCall, that calls a function
until it succeeds. A back-off iterator (a generator function that yields
intervals) can be specified to customize the interval between retried
calls. When/if the back-off iterator raises StopIteration the attempt to
call the function is aborted. An instance of the RetryingCall class
provides a start method that returns a Deferred that will fire with the
function result or errback with the first failure encountered.
Usage of the class is described in the following blog post: http://blogs.fluidinfo.com/terry/2009/11/12/twisted-code-for-retrying-function-calls/