fileset-rebase: documentation + only relativize hrefs if xml:base present
px:fileset-move: changes the xml:base of the fileset, without updating the hrefs

"original-href"-attributes will be added for files that exist on disk. No files will be physically moved, that's what px:fileset-store is for.
added px:fileset-move to the fileset xproc library
added missing fileset-move.xsl

(px:fileset-move is imported from the nordic-epub3-dtbook-migrator project and I forgot to copy the XSLT)
Merge pull request #51 from daisy-consortium/fileset-move

Close #51