Refactor ThingWrapper serializations

### Why
The standardization of a ThingDescription API is not yet available, so we are going forward with the existing implementation from [Mozilla WebThings](https://iot.mozilla.org/wot/) as a practical solution.

We wish however to remain compatible with ThingDescription draft specifications.

### What
- [ ] implement serialization of WebThing to simple JSON (see [this example](https://iot.mozilla.org/wot/#example-1-a-simple-thing-description)) - this is the default of WebThing

- [ ] implement serialization of WebThing as JSON-LD using the [JSON Schema for TD Instance Validation](https://w3c.github.io/wot-thing-description/#json-schema-for-validation)

- [ ] implement serialization of WebThing as RDF Triples using [ThingDescription Ontology model](https://www.w3.org/2019/wot/td)

### Notes
- A client must switch between the representations using a query string: ?format=json, ?format=jsonld, ?format=rdf
- JSON-LD and RDF representations should be interchangeable when converting to and from using [this tool](http://rdf-translator.appspot.com/) or Python RDFLib to read an RDF graph and serialize it to JSON-LD