Remove basic authentication handler

Also removes the json_check_authentication filter. All authentication handlers should switch to the determine_current_user filter added in https://core.trac.wordpress.org/changeset/27484

Adds a json_authentication_errors filter to allow reporting a WP_Error back to the API on failed authentication.

Basic Auth is now available via a [plugin](https://github.com/WP-API/Basic-Auth) instead.

Fixes #37.