pidstore: provider status sync and celery tasks

* Adds feature to synchronize the status of a persistent identifier
  with the provider. Useful for cases where the status of an
  identifier might have been modified in an external service.

* Adds namespace prefix to configuration variables.

* Adds celery tasks to registering DataCite DOIs for records.

* Fixes some PEP8 errors.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>