First pass at hypermedia infrastructure

This adds an add_link method to WP_JSON_Response, allowing hypermedia
inks. You can then embed responses by requesting ?_embed
Convert errors in embedded to correct data
Move prepare_response into response_to_data
Use new-style links in the response
Only allow certain links to be embedded
Always pass context=embed when embedding
Optimize Request constructor for common usage

This makes it nice and easy to make a request internally. Simply:

	$server = new WP_JSON_Server();
	$request = new WP_JSON_Request( 'GET', '/posts/4' );
	$response = $server->dispatch( $request );
Rename file to better communicate purpose
Introduce a new dedicated Controller for taxonomies
Avoid strict notices by making these requirements looser
Pluralize for consistency
Basic test coverage for taxonomy route registration
Fix undefined variable
Fix invalid variable
Move taxonomy test coverage to its own class
Test preparing a taxonomy

Oh yeah red lines!
This now forever shall be known as the `WP_JSON_Terms_Controller`
Restore our term test coverage
Normalize error codes for terms
Routes should include the namespaces