Merge pull request #78 from oreillymedia/htmlbook-xsl

Modularized TOC heading value generation to create separate toc-title template.
Addded support for nesting <img> tags as children of <section> elements, as well as Part <div>s.
Merge pull request #79 from oreillymedia/xmlschema

Addded support for nesting <img> tags as children of <section> elements as well as Part <div>s.
Added support for extra <div class="border-box"> in figure postprocessing; see #80.
Merge branch 'master' of github.com:oreillymedia/HTMLBook into xmlschema
Merge pull request #81 from oreillymedia/xmlschema

Added support for extra <div class="border-box"> in figure postprocessing
Added $custom.chunk.wrapper param that lets you specify a custom wrapper to put around chunk content.