It's useful to be able to list all the (flattened) keys of a
ContextList, to help you figure out why the variable that's supposed
to be there is not.
No .values() or .items() added as the definition for those aren't clear.
The patch is Chris Wilson's patch from pull request 1065 with some
modifications by committer.
SuspiciousOperations have been differentiated into subclasses, and
are now logged to a 'django.security.*' logger. SuspiciousOperations
that reach django.core.handlers.base.BaseHandler will now return a 400
instead of a 500.
Thanks to tiwoc for the report, and Carl Meyer and Donald Stufft
for review.
