WP_JSON_Controller POC

Use @joe_hoyle's proposal from #551 to register the new taxonomy terms controller introduced in #549
@TimothyBJacobs You can call a static function as if it were a normal method on the class, but you can't do the other way around. The advantage is you don't _need_ to create an instance if we make the method static. If you subclass any of these classes, you have the choice if you want to create instances and use state within that object, and the fact the parent methods are statically declared won't affect you at all. It's not the same if we make the base classes require state, and therefor all subclassing will require instantiation.