John J. Lee writes: "the patch makes it possible to implement
functionality like HTTP cookie handling, Refresh handling,
etc. etc. using handler objects. At the moment urllib2's handler
objects aren't quite up to the job, which results in a lot of
cut-n-paste and subclassing. I believe the changes are
backwards-compatible, with the exception of people who've
reimplemented build_opener()'s functionality -- those people would
need to call opener.add_handler(HTTPErrorProcessor).

The main change is allowing handlers to implement
methods like:

http_request(request)
http_response(request, response)

In addition to the usual

http_open(request)
http_error{_*}(...)
"

Note that the change isn't well documented at least in part because
handlers aren't well documented at all.  Need to fix this.

Add a bunch of new tests.  It appears that none of these tests
actually use the network, so they don't need to be guarded by a
resource flag.