[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
Adding the `@base` was already conditional on the trueness of `jsonld_baseurl`. (so in fact it would not even end up setting `@base=""` by default like you feared)