Add ability to set SMTP envelope addresses

I am working on a program that is called from a mail server, and re-injects the message back into the mail queue to be sent to multiple recipients (using a Postfix content filter).  The problem is that this library forms the RCPT TO: information from the message headers, which in this case, will not work (the goal is for the message to arrive untouched).  There should be an option to pass SMTP-level message options to the transport to handle situations like this.

I currently have something like the following:

```
$email = file_get_contents("php://stdin");
$headers = $body = '';
Zend\Mime\Decode::splitMessage($email, $headers, $body);

// Setup SMTP transport
$transport = new Zend\Mail\Transport\Smtp();
$options = new Zend\Mail\Transport\SmtpOptions(
        array(
    'name' => php_uname('n'),
    'host' => '127.0.0.1',
    'port' => 10025,
        )
);
$transport->setOptions($options);

$message = new Zend\Mail\Message();
$message->setHeaders($headers);
$message->setBody($body);
$transport->send($message);
```

This pull request will allow you to do:

```
    $message = new Zend\Mail\Message();
    $message->setHeaders($headers);
    $message->setBody($body);
    $envelope = new Zend\Mail\Transport\Envelope(array(
        'from' => 'mail-from@address.com',
        'to' => 'rcpt-to@address.com',
            )
    );
    $transport->setEnvelope($envelope);
    $transport->send($message);
```
Please use the getter directly. Also check if the `envelope` exists first