Fixed some questionable syntax.

In some PHP/libxml versions, $node->xpath(...)[n] is valid syntax, and
in others it isn't. The instance of it here is replaced with a longer
but more portable construction.
Added handling for deleted records.

The BL.ADS set on the DataCite OAI-PMH endpoint has multiple deleted
records. For now we just ignore them, but they ought to trigger removal
or 'tombstoning' in the registry.