Code review comment for lp:~leonardr/launchpadlib/retry-on-invalid-token

Here's an incremental diff: http://pastebin.ubuntu.com/544534/

I've written a framework that lets me send fake HTTP responses to launchpadlib, allowing me to finally do a (sort of) end-to-end test of login_with(), as well as demonstrate what happens if Launchpad suddenly starts sending 401 responses to launchpadlib's requests. I had to do a tiny bit of refactoring of the LaunchpadOauthAwareHttp._request method, and I moved several helper classes from test_launchpad into testing/helpers.py (because they're now used by more than one test module), but almost all of this code is new tests.