Use OA context from w3.org
Set jsonld_baseurl default to None
Semantify user

Let's keep the default very generic, for example we don't even know if
users are Persons. Implementors can subclass Annotation to add more user
info (as with any of the properties).
Content negotiation: handle application/ld+json
Add test for json and ld+json mimetype negotiation
Set jsonld_baseurl in store.py
Add test for JSON-LD content