Some reorganization of templatetags html-snippet files.

Also:
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

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