Update dtbook-to-epub3.xsl

Hi Jostein,

I've been making minor changes in my local scripts to nudge some of the transformations in the batch script along, and hopefully see some more issues in the output. Ones that seem to work I could commit to this fork for   you to look at... in case any of it could be useful. 
Let me know if this is a problem.
By the way, I also have a windows version of your original bash script that's more-or-less stable. Do you think that would be worth making available on the batch branch?

/Tam
I think we want to move pagenums into their first preceding cell. Since pagenums can only be siblings to table rows (`tr`) and not cells (`td`/`th`), the pagenums will always end up in the last cell in a row. So we need to check if the current cell is the last cell in the row, not the first cell.