Added dom4j dependency in the parent pom.xml

We should start migrating away from org.w3c.dom classes into org.dom4j
classes and this is the first step.
Changed the OAI-PMH data loader to use the dom4j library

This change generalizes the data loader since it allows us to
effectively ignore the XML namespaces. It also simplifies the code and
the error conditions we need to check for.
OAI-PMH data loader now correctly loads one record if passed a correct data loading spec