Created DescriptiveMetadataDatastream class, copied from ActiveFedora::QualifiedDublinCoreDatastream
DulHydra content models now use new ds for descMetadata
Closes #814
Merge pull request #846 from duke-libraries/814-dmd-datastream

Replaced ActiveFedora::QualifiedDublinCoreDatastream with DulHydra::Datastreams::DescriptiveMetadataDatastream