WP_JSON_Controller POC

Use @joe_hoyle's proposal from #551 to register the new taxonomy terms controller introduced in #549
@danielbachhuber I think that would depend what purpose we want abstract. If it's to enforce the developer to create the method, and therefor throw a fatal error if it hasn't been declared - we'd need static still.

If on the other hand we just want a response, then it would cover that use case. However, that also depends if just subclassing / instantiating the object would also register the endpoint. If you also have to register your endpoint manually anyway - you wouldn't add the methods you hadn't implemented anyway.