SF bug #681003: itertools issues

* Fixed typo in exception message for times()
* Filled in missing times_traverse()
* Document reasons that imap() did not adopt a None fill-in feature
* Document that count(sys.maxint) will wrap-around on overflow
* Add overflow test to islice()
* Check that starmap()'s argument returns a tuple
* Verify that imap()'s tuple re-use is safe
* Make a similar tuple re-use (with safety check) for izip()