Replaced registration URL for WordPress integration

Use WordPress function `wp_registration_url` for fetching registration URL while still keeping compatibility with older WordPress (bellow 3.6) that do not have this function.

qa-include/util/external-users-wp.php

@@ -36,7 +36,7 @@
 	{
 		return array(
 			'login' => wp_login_url(qa_opt('site_url').$redirect_back_to_url),
-			'register' => site_url('wp-login.php?action=register'),
+			'register' => function_exists('wp_registration_url') ? wp_registration_url() : site_url('wp-login.php?action=register'),
 			'logout' => strtr(wp_logout_url(), array('&' => '&')),
 		);
 	}
@@ -163,4 +163,4 @@
 
 /*
 	Omit PHP closing tag to help avoid accidental output
-*/
\ No newline at end of file
+*/