Fix for issue 278

px:fileset-add-entry and px:fileset-join only relativizes the hrefs if the fileset has a xml:base attribute.

Issue tracker: https://code.google.com/p/daisy-pipeline/issues/detail?id=278

@marisademeglio : see if this does what you requested
@rdeltour : I hope it is ok to ignore the base URI of the d:fileset if there is no xml:base attribute - I think it's the easiest way to have "base-less" filesets