support for writing Dublin Core metadata added #18
included adding ReferenceMeta as a class and convincing the meta class to handle
both the LiteralMeta and ReferenceMeta based on xsi:type, which has been a bit troublesome!

xsi:type is a bit annoying since I'm not really clear how the XML package is handling the
namespace on this attribute.  Sometimes it seems implicit and sometimes explicit, so we now
try both cases.

The CC0 reference link is given as a ReferenceMeta.  Should possibly have a LiteralMeta with
the dc:rights associated with it?

Test file for checking these results also added (test_meta.R).  Could be extended (particularly
once we have dedicated RDFa parsing approaches rather than just querying the S4 classes?  Or
write functions that extract all the meta-class nodes from the S4 at very least.

Do these meta nodes need more/better referencing and ids?