First pass at HTMLBook TOC autogeneration

I've added a htmlbook-xsl directory, which will contain tooling for HTMLBook-specific transforms.

This pull request includes common.xsl, which contains some common utility templates, and tocgen.xsl, which is a first pass at stylesheet handling to autogenerate an HTMLBook-conformant Table of Contents. Note that some refinements are still necessary (most notably, handling to enable you to specify TOC section depth), as is more testing, but I think it's in good enough shape to merge into master now.