Added compatibility with laravel's Mailer class.

All that is needed to do to use it with Laravel is:

``` php
\Mail::setSwiftMailer(Swift_PostmarkTransport::newInstance('API KEY HERE'));
```

A minor change was needed to make it work with Laravel.
When sending an HTML + plain-text email, Laravel uses `multipart/alternative` as the mimetype, which was throwing things off.

Also made the `getMIMEPart()` function a little better, by returning when it finds the part it's looking for rather than running through the entire loop.