Turns headers into mailgun params

This lets you use the [mailgun headers](http://documentation.mailgun.com/user_manual.html#sending-via-smtp) instead of using the additional hashes that mailgun_rails added on to `Mail::Message`.  I've also added the ability to set the [option params](http://documentation.mailgun.com/api-sending.html#sending).

It might make sense to deprecate the hashes and move over to using the headers since you can set the same value multiple times.  This is important for various options in mailgun like the `o:campaign` which can be set 3 times.  Of coarse this wouldn't be possible using the current client.  It would have to be moved over to using [`mailgun-ruby`](https://github.com/mailgun/mailgun-ruby).