* Add dependency on `WebHelpers`
* Remove table structure from `todo_list.pt`
* Add custom `TodoGrid` subclass of `webhelpers.html.grid.ObjectGrid`
  * Implement `generate_header_link`
* Update `tag_view` and `list_view` to honor the `sort_col` and
  `sort_dir` variables generated by `TodoGrid`
* Make `pretty_date` a stand-alone function instead of a method
* Make `past_due` a bit more defensive