Discover metadata for unknown sources.

This gets a list instead of a string/URI value, at least when trying on http://brugd.ctrl-c.liu.se/~petli/examples/ttcfont.xhtml:

      {
        "statementLink": [
          "http://creativecommons.org/licenses/by/3.0/"
        ],
        "typeLabel": "license",
        "propertyName": "policy",
        "value": [
          "http://creativecommons.org/licenses/by/3.0/"
        ],
        "mappingType": "Exact match",
        "sourceFormat": "rdfa"
      }

The annotation lack a typeLink too.