Added compatibility with laravel's Mailer class.

All that is needed to do to use it with laravel is \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.