Makes fedora 3 datastream content available.

The bug was that the Connector implementation (Fedora3FederationConnector) failed to override getBinaryValue method.  Though the Document returned by this connector (and others) includes an ExternalBinaryValue impelementation as the jcr:data property to the jcr:content document, when it's translated into a node a call is made to the connector to get the ExternalBinaryValue, and if null is returned, silently replaces the existing binary value with an empty one.

This was rather hard to track down, but the fix includes quite a bit of refactoring and improvements to the test rigging.