Webhook tests define a local signal receiver function,
so connect it using the `weak=False` option to set
a good example.
(This isn't technically needed in the tests: the test receivers
are only connected while their definitions are still in scope,
so they couldn't possibly be garbage collected. But it doesn't
hurt, and it's good practice in case the test code gets copied.)
Also update the webhook docs to have a direct link to
Django's "listening to signals" info.
Second attempt to address possible test sequencing issue around tests that alter settings. (Failures in Travis tests not reproducible locally.)
Back-ports override_settings from Django 1.4 for compatibility with Django 1.3.
