Remove duplicate imports of django.conf.settings.

See
<https://github.com/pythonforfacebook/django-facebook/commit/2c75177e5061123387dfc637eb382de41d899264#commitcomment-4370000>.
Change error raising in facebook_init.

"token" does not exist in template.Node.init(), and we know that the
name of the tag is "facebook_init".
Various fixes based on linting with flake8.