Merge pull request #42 from oreillymedia/htmlbook-xsl

First pass at HTMLBook TOC autogeneration
adding more sample content
Merge pull request #43 from oreillymedia/content

adding more sample content
Allow empty <nav> element placeholder for TOC. Add support for cover image specified in <figure> element in <body>.
Updating sample document for latest HTMLBook specs.
Fixing merge conflicts.
Merge pull request #45 from oreillymedia/xmlschema

Adding cover spec to HTMLBook schema
Base starting point for indexgen stylesheet: "basic" index generation logic from xhtml/autoidx.xsl in docbook-xsl project.