Qualified Dublin Core metadata format for OAI-PMH

Hi,

I added a qualified Dublin Core metadata format (qdc). This is needed to expose Qualified Dublin Core with the this plugin. 

This format is not standardized, but used by some repositories, as DSpace, ContentDM, or a mediawiki extension ProofreadPage (https://wikisource.org/wiki/Special:ProofreadIndexOaiSchema/qdc). The schema comes from the Science & Technology Facilities Council of the United Kingdom (see http://epubs.cclrc.ac.uk/xsd/qdc.xsd). In fact, this is almost the same than the unqualified Dublin Core, except that `dcterms:` is used for qualified elements.

I added an option too that allows to merge refined elements into the fifteen default elements for OAI-PMH, so all elements are harvestable via the standard oai_dc metadata format.

This pull request needs the new filter added via https://github.com/zerocrates/OaiPmhRepository/pull/10.

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management