Example of px:fileset-add-ref

@rdeltour I think you created px:fileset-add-ref, but it doesn't seem to be used anywhere, and it's not listed on http://code.google.com/p/daisy-pipeline/wiki/FileSetUtils. How did you intend it to be used?

In my [script for generating EPUB 3 with braille rendition](https://github.com/snaekobbi/pipeline-mod-braille/pull/6) (https://github.com/snaekobbi/pipeline-mod-braille/pull/6) I'm renaming several files in a fileset and I need to update cross-references between those files. I thought it was a possibly use case for px:fileset-add-ref but I'm not sure it is?