Use RDF equality when diff'ing user-provided triples against stored triples
Merge pull request #552 from fcrepo4/jena-graph-containment

Use Jena's notion of graph containment instead of our strict equality
Implement a Converter<Resource,FedoraResource> to replace the Node converter
Use FedoraResource#getContainer/getChildren when serializating resources

Also, update getContainer + getChildren to be pairtree aware.
Add Tombstone resource when a resource is deleted, preventing the re-use of URIs until the tombstone is also removed