DS-2061: Linked (Open) Data support for DSpace

This commits adds a module dspace-rdf. The module will convert the
metadata stored in DSpace into RDF. The converted data will be provided
as serializations of RDF and a SPARQL endpoint.

This PR should not be merged yet, as there are some things left to be done. I create this PR now to share the code as early as possible. I would appreciate any comments and if possible help to get it merged into DSpace.

You can find more information on this contribution here:
 * https://wiki.duraspace.org/display/~pbecker/Bringing+DSpace+into+the+Semantic+Web
 * https://jira.duraspace.org/browse/DS-2061
 * http://www.pnjb.de/uni/diplomarbeit (currently only in German)