Implement a JSON-LD schema importer

[bergie]

In addition to the current schema format outlined in #141, we should also support importing JSON-LD -encoded schemas. An example can be found from http://drupal.org/node/1831286#comment-6690390

[linclark]

The important thing about the schema described in that issue is that it uses RDFS, which is a W3C standard for expressing RDF schemas. This could be expressed in JSON-LD, but it could also be expressed in RDFa.

It's worth noting that RDFS could be used to consume Schema.org as well. Schema.org's official vocabulary metadata uses RDFS (expressed in RDFa). They have also started using RDFS for vocabulary imports.