Support for directories and page crumbs

This changeset adds support for displaying a directory as a series of nested lists of the files inside it. It also adds "page crumbs" (links to each component of the path) to each page and directory template.

Other changes:
* Added a 'SORT' option in config.py; set it to 'title' or 'url' to control the order in which index() sorts pages.
* Changed the fuctionality of the "home" page. If there exists a page tagged 'HOMEPAGE', that's used for the '/' url; otherwise, the full tree of lists is displayed.