Change settings/base.py to use 'BASE_DIR' convention.

BASE_DIR is now a standard convention for project root dir, cf.
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.