fix for issue 278
https://code.google.com/p/daisy-pipeline/issues/detail?id=278
Merge branch 'master' into issue278
Updated to work with fileset changes on this branch

file @hrefs are absolute; ref @hrefs are relative to the file. supports
multiple ref elements per file.
fixed issue 276
Merge branch 'issue278' of https://github.com/daisy-consortium/pipeline-modules-common into issue278
Merge pull request #6 from daisy-consortium/issue278

Fix for issue 278