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.
Remove basic authentication test

This test will intentionally fail now that basic auth has been removed.
Merge pull request #152 from WP-API/remove-basic-authentication

Remove basic authentication handler.  Closes #37 and #152.