First pass at adding REST API compatibility

civicrm.php

wp-rest/Autoloader.php

wp-rest/Civi/Mailing-Hooks.php

wp-rest/Controller/AuthorizeIPN.php

wp-rest/Controller/Base.php

wp-rest/Controller/Cxn.php

wp-rest/Controller/Open.php

wp-rest/Controller/PayPalIPN.php

wp-rest/Controller/PxIPN.php

wp-rest/Controller/Rest.php

wp-rest/Controller/Soap.php

wp-rest/Controller/Url.php

wp-rest/Controller/Widget.php

wp-rest/Endpoint/Endpoint-Interface.php

wp-rest/Plugin.php

wp-rest/README.md