Add getChildren() signature in FedoraResource...

 - Implement it in FedoraResourceImpl.java
 - Expose the hierarchy levels through the IdentifierConverter and IdentifierTranslator on behalf of the getChildren() implementation.
 - Integrate the getChildren() method in FedoraResource to generate the hierarchy rdf triples for REST web UI.
 - Log the potential null children nodes and return as suggested.
 - Implement FedoraResource.getParent() for transparent hierarchy support.

Resolves: https://www.pivotaltracker.com/story/show/71941366