Ensure all metadata fields are exported through OAI with oai_dc.

Some fields were not exported which can be a problem when harvesting
from another repository.
Don't export dc.description.provenance.

The default is to hide this metadata (see
metadata.hide.dc.description.provenance option in dspace.cfg).
Update metadata.hide.dc.description.provenance option comment.

This option no longer affects OAI-PMH.
Merge pull request #258 from rivaldi8/oai_dc-export-fix

DS-1602 fix