Allow attributes on dcterms elements in Atom entries

Attributes of DC terms sent back in a deposit receipt are available through SimpleXML -- it would be nice to be able to include attributes when creating Atom entries for deposit as well. 

E.g., when the OJS Dataverse plugin creates Atom entries to deposit studies in a Dataverse, attributes on the isReferencedBy element can be used to construct a persistent URI to the OJS article associated with the study in the study's cataloguing information (see [Create a study with an Atom entry](http://thedata.harvard.edu/guides/dataverse-api-main.html#create-a-study-with-an-atom-entry-xml-file)).