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
`dtbook:pagenum[1]`on this line only copies the first preceding pagenum. There can be multiple preceding pagenums. Removing the `[1]` should fix this I think.