Do not pop from config['extractors']

Otherwise the simple_cookie extractor is removed from the global
config.

This code was present from an earlier version that added to the
extractors config at the module global level. It has since moved
to just one method.
Allow unicode usernames in auth cookies.

Until now usernames that are unicode were not being
properly encoded when `tiddlyweb_user` cookies were being
created. This change updates both the cookie creation and
simple_cookie extractor such that the value of any cookie
will be considered encoded (unicode -> utf-8 -> uri escaped).

For most existing implementations this shouldn't mean anything:
you couldn't use unicode until now so nobody encouraged it, so the
problem didn't show up. Those existing services will have the same
cookie values encoded or not. However Tank can allow unicode usernames
(because some of the upstream auth providers do) so this bug was
tickled. I'm hoping Tank will tickle more.