- Added support for empty title elements.

- Added new mechanism for better handling of assignment due dates. ...

- Made more consistent use of xsl:text.

- Removed a title template that could never get called.

- Moved sectioning and titling templates into separate modules. ...

- Reimplemented "generate-subs" template to use a sequence loop rather than
  recursion.
- Refactored the way section depth is calculated.