adding Dataverse plugin for OJS 2.4.4

FYI -- we're going to need to do another pull request and freeze a commit for the swordappv2 submodule in pkp-lib (the plugin currently requires this patch to be run after OJS is installed: https://github.com/axfelix/dataverse/blob/master/swordappv2.diff), but Jen and I will follow up with them to make that happen and I'll open another pull request to update the submodule.