Extend CoreNLP XML parser with coref chains
Absolute(ish) token ids when reading from CoreNLP

Systematically prefix token ids with their sentence id.
It's not so much for fear of losing the information as getting
some consistency in the treatment of these ids.
Tidy code up a smidgen
Return coreference chain annotations from corenlp
Remove CoreNlpDocument.annotations

Can't return everything as it wouldn't be very sensible
(would it?), so might as well just nuke the function to
avoid confusion
Avoid collapsed dependencies in CoreNLP for now.

They're nifty but not tree-shaped:
http://nlp.stanford.edu/software/stanford-dependencies.shtml