daisy202-to-epub3: more log messages

 * added more log messages to daisy202-to-epub3, both for easier debugging and more insight into what is happening when running the script
 * replaced pxi:fix-textrefs with a XSLT template; XSLT templates perform better than recursive p:viewports
 * replaced a p:count/p:choose/re-pipe assertion with px:assert
daisy202-to-epub3: include dc:identifier in navigation document

 * the dc:identifier is needed for determining the dtb:uid in nav-to-ncx.xsl
 * also supports retrieving the language from `/*/html:head/html:meta[lower-case(@name)='dc:identifier']` in addition to `/*/@lang` and `/*/@xml:lang`
Merge pull request #40 from daisy/daisy202-to-epub3-improved-log-messages