EPP Logout frame

The library is missing a frame that terminates the EPP connection. I'd suggest adding a 
Command/Logout.php

```
<?php

/**
* @package Net_EPP
*/
class Net_EPP_Frame_Command_Logout extends Net_EPP_Frame_Command {
    function __construct() {
        parent::__construct('logout');

        $this->clID = $this->createElement('clID');
        $this->command->appendChild($this->clID);
    }
}
?>
```
