Merge pull request #89 from oreillymedia/htmlbook-xsl

Support for PIs for navigating among HTML chunks
Merge pull request #90 from oreillymedia/htmlbook-xsl

Making header for cover.html page consistent with other chunked pages for EPUB.
Fixed error in note about <canvas>.
Fixed validity error in htmlbook.html sample; subtitles must have  corresponding data-type attribute; see #92
Merge pull request #93 from oreillymedia/samples

Fixed validity error in htmlbook.html sample; subtitles must have data-type
Refinements to cover handling.