Removes stub method to purge cache out ouf EZIDConnector.
Added method to register DOIs. Added Debug information.
Changed ExceptionHandling in DOIIdentifierProvider.

DOIIdentifierProvider and DataCiteConnector will throw more
IdentifierExceptions and less other Exceptions, like RuntimeException,
IllegalArgumentException or IllegalStateExceptions.
IdentifierServiceImpl catches IdentifierException but not the other
ones. If IdentifierServicesImpl catches an IdentiferException it may go
on with the next IdentifierProvider and not the complete user action
will stop.