[WIP] First attempt at using Open Annotation Data Model

Annotations can now be serialised into JSON-LD formatted RDF, following
the data model spec: http://www.openannotation.org/spec/core/

Addresses #74
Feels better to leave this out to me. Otherwise we might end up setting a `base=''`. I don't know what JSON-LD processors are supposed to do with that. Let the caller add it to the context if necessary, I think.