ToDo note for XREF gen.
Support for new param $generate.root.chunk that lets you decide whether or not to generate a root chunk (i.e., index.html)
Fixed bug in full-output-filename logic, and revised to accommodate new $generate.root.chunk param
Revised EPUB handling to accommodate new generate.root.chunk param.
Actually generation order does not matter here, as logic is now written.
Merge pull request #60 from oreillymedia/htmlbook-xsl

Better handling of EPUB frontmatter generation