fix exceptions namespace

when loading the library with composer, the exceptions namespace is not included in the autoloading, as the library include path only loads `ZendService\\Apple\\Apns\\`, and the exceptions are under `ZendService\\Apple``. 

```php
PHP Fatal error:  Class 'ZendService\Apple\Exception\InvalidArgumentException' not found in /var/test/vendor/zendframework/zendservice-apple-apns/library/ZendService/Apple/Apns/Client/AbstractClient.php on line 66

Fatal error: Class 'ZendService\Apple\Exception\InvalidArgumentException' not found in /var/test/vendor/zendframework/zendservice-apple-apns/library/ZendService/Apple/Apns/Client/AbstractClient.php on line 66
```