workflows: improve holdingpen data display

* Adds a new function to BibWorkflowObject that tries to
  return a well formatted representation of the data.

* Improves the formatting sequence when displaying data from
  workflow objects in Holding Pen to not cause exceptions when
  data is non-XML. Data is now shown as a pretty string when
  not formattable by formatter or legacy.bibfield.
Merge pull request #130 from jalavik/workflows-hp-format

workflows: improve holdingpen data display