Proof-of-concept federation over fedora 3 content.

This should address tracker ticket 55769132 as well as comments from the previous two pull request.

Test coverage over the *.rest package is known to be sparse as that the "proof-of-concept" part and is basically just a thin wrapper over the FedoraClient code. Furthermore, that will be tested by forthcoming integration tests that are part of another ticket.

Furthermore, though it was suggested that I replace "jcr:content" node names with "fcr:content", further investigation reveals that in org.fcrepo.kernel.utils.addJcrPropertiesToModel() the presence of a child with the name "jcr:content" is required by that code to add a fedora:hasContent relationship.

That said, content downloading doesn't work yet, but it is acknowledged in the README and will be the subject of future (more focused) work.