Add Open Annotations vocabulary

[dazza-codes]

vocab_sources = {
  oa:  {
    uri: 'http://www.w3.org/ns/oa#',
    source: 'http://www.openannotation.org/spec/core/20130208/oa.owl',
    strict: false
  },
}

Actually not sure whether it should be strict: false or true; have to clarify that point. Otherwise, it works with rake gen_vocabs.

[gkellogg]

This might be better in the rdf-vocabulary gem. Ultimately, most gems will move over there: https://github.com/projecthydra-labs/rdf-vocab.

[dazza-codes]

Moving this concern to rdf-vocab.