fix handling of XML namespaces

Add better support for namespace prefixes. Now parser marcxml.pm can handle records with prefixes like "<slim:record>" or "<marc:record>". 

Files changed/added:
  * marcxml.pm: ignore namespaces, use of "getChildrenByLocalName()" to get child nodes
  * SRU.pm
  * marcxml_ns.xml: sample record with different prefixes
  * namespaces.t