Added export_date attribute and db column to OpmlExportJobState model.
When an OpmlExportJobState instance is updated with state "SUCCESS", the export_date field must be given a value (usually the current datetime).
If the OPML export for the user has state "SUCCESS", the JSON returned has the export_date attribute.

Previously the updated_at attribute was used, but I don't think it's totally trustworthy for my ends.