Added an XML namespace manager for the XPathRecord

Added a class that manages the XML namespaces for the
XPathRecords. For the moment, the namespaces are hardcoded for the
responses to the oai_dc namespaces, used by the OAIPMHDataLoader, but
it will be transformed to dynamically load the namespaces existing in
an XML string.
Added a first version of the OAI-PMH data loader.

What still needs to be done is the loading of a single record.