WP_JSON_Controller POC

Use @joe_hoyle's proposal from #551 to register the new taxonomy terms controller introduced in #549
@danielbachhuber That would definitely solve the type enforcement, but extending would still be a bit difficult.

@joehoyle I see your point, in terms of the callback not needing to be part of the class structure, and just needs to be callable.

We could benefit, though, from having a class structure.

We could do something like this.

````
    if callback is array AND object instance of base class
        proceed like normal
    otherwise
        we can attach the passed callable function into a class called WP_JSON_Controller_Callable
````
This way we can store all of the routes easily inside of an array in the server, and call the correct methods depending on what request was passed.

Does that make sense?

Then it would also make perfect sense for the default for those methods to return a 403 or 405.