Improve EPUB metadata merge

 * OPF metadata is obtained by merging the `metadata` sequence port
 * merging is done in XSLT (extensively tested)
 * default metadata is piped as the last documenet to the merge step

@josteinaj feel free to review if you want, or directly merge. The XSLT is extensively tested, see tests and inline 'documenting' comments for what it does.