Terms Controller Redux

* [x] Restore terms endpoints with tests
* [x] Restore taxonomy endpoints with tests
* [ ] Add create and delete methods to term endpoint.
@danielbachhuber The way we've done it is using arrays, so...

```php
register_json_route( 'wp', '/terms/(?P<taxonomy>[\w-]+)/(?P<term>[\w-]+)', array(
	array(
		'methods' => array( 'GET' ),
		'callback' => array( $controller, 'get_item' ),
	),
	array(
		'methods' => array( 'POST', 'PUT', 'PATCH '),
		'callback' => array( $controller, 'update_item' ),
	),
	array(
		'methods' => array( 'DELETE' ),
		'callback' => array( $controller, 'delete_item' ),
	),
) );
```