Working Mailgun transport adapter

First, this code was just written and documented, so feel free to refactor if you see stuff that needs to be done in a better way. Matter of fact, hit me with the changes and I'll make them for you so we can learn the Li3 Way(TM)...

Ok we got support for mailgun, also checks for html.php and text.php. We managed to hook in support for multipart messages as well, so you can send both; and we also plugged in tags so you can tag your messages so they are grouped in the reporting interface at Mailgun.

I can commit all our changes here and make pull requests if that's how you like to roll. Thanks for li3_mailer, hope to be more active with it in the future :-)

-- Mitch