Add the getUsername method in our own interface

The bundle still relies on a username even though Symfony renamed the
method used for the user identifier. So we need to have the method in
our own interface instead of relying on the inherited method.

Author: stof

Model/UserInterface.php

@@ -53,6 +53,11 @@ interface UserInterface extends CompatUserInterface
      */
     public function getId();
 
+    /**
+     * @return string
+     */
+    public function getUsername();
+
     /**
      * Sets the username.
      *