[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
I think it's nice to have some default setting for it, so it's immediately obvious that it can be set, there's a place for its documentation, it shows up in tab completion in IDEs, etcetera. Setting it to None seems to be the elegant approach here.