Make verify() follow the specs and return null

index.js

@@ -91,8 +91,8 @@ hotp.verify = function(token, key, opt) {
 		}
 	}
 
-	// If we get to here then no codes have matched, return false
+	// If we get to here then no codes have matched, return null
-	return false;
+	return null;
 };
 
 var totp = {};