Add metadata retrieving from XML

As an uploader, I want to process metadata provided in XML by the GLAM.

GLAMs may provide their metadata in XML format.

Here we add two methods:
- retrieve_metadata_from_xml iterates through the XML
  based on a given node name to look for
- get_metadata_from_xml_element is a 'best-effort' metadata element
  which more or less dumps all the subelements in a dictionary
  (assuming the XML file is in flat format)