Define extension mechanism for defining Thing types about api

[benfrancis]

Note that JSON Schema is mostly about validating existing JSON data, whereas JSON-LD is about attaching semantic meaning to JSON using lightweight Linked Data markup. The former could provide a way to validate that a Web Thing Description is valid and the latter can provide an extensibility mechanism to allow anyone to define their own Web Thing types and provide a semantic context through which to interpret their meaning.

In future it might be useful to borrow the data types definition mechanism from JSON Schema for properties, action arguments and event payloads.

We should also define a JSON Schema to validate Web Thing Descriptions and a default Web Thing JSON-LD context.