Mark as Countable interface

Client can use native php count function

src/Mailbox.php

@@ -6,7 +6,7 @@
  * An IMAP mailbox (commonly referred to as a ‘folder’)
  *
  */
-class Mailbox implements \IteratorAggregate
+class Mailbox implements \Countable, \IteratorAggregate
 {
     private $mailbox;
     private $name;