Add graph to initializer
Remove :crossref from fallback
Use Entry#identifier in #children and #parent
Add agent to initializer

key of #agent could be BibTeX::Name. And this would be always a different object. This is why we use then key#to_hash as the key.
Fix calls to #agent
Fix #parent and #children

As we did not check, if an Entry was already in our graph, the converter would run infinitely.