txretry provides a Twisted class RetryingCall that will attempt to call a
function until it succeeds. A back-off iterator (an interator yielding
intervals) can be provided to set 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/