Change API key on existing instance

[ghost]

I think, it would be useful to have an option to change the API key on an existing instance. We are using your class within a batch job and need to change "Servers" back and forth, depending on the message we are currently processing. For now, we have just added these few lines:

/**
* Set or change the Postmark API key
*
* @param string $apiKey Postmark API key
* @return Mail_Postmark
*/
public function &setApiKey($apiKey)
{
  $this->_apiKey = $apiKey;
  return $this;
}