Merge pull request #63 from oreillymedia/htmlbook-xsl

Added support for autogenerated labels on headings/captions
Updating title page

See https://github.com/oreillymedia/HTMLBook/issues/64.
Allow mixed content for <video> and <audio> elements, for the purposes of fallbacks.
Merge pull request #69 from oreillymedia/xmlschema

Allow mixed content for <video> and <audio> elements
Merge branch 'master' of github.com:oreillymedia/HTMLBook into htmlbook-xsl
Removing variables from xsl:key "use" attributes to be in compliance with XSLT 1.0 spec.