Tweaks to settings/base.py

BASE_DIR is now a standard convention (same idea, standard name).
https://github.com/django/django/blob/stable/1.6.x/django/conf/project_template/project_name/settings.py#L13

Also add a comment re: HybridTemplateMixin, since otherwise the fact we don't
include the banana templates dir is pretty confusing to any newcomer.