Parser fixes

- Fix: It seems, that we do not get an issue key in every case. The
  parser will now use "" as dummy values. This should be reconsidered.

- New function map-issue-fields to map a function over all present
  fields in an issue (including key/id if desired)

- New function issue-field to access the value associated with a single
  issue field. Also works with key/id.
Fix: the issue key was not properly passed on