Mailgun EU Region Domains

[richardherbert]

As currently coded the Mailgun protocol assumes a US region domain. If the domain is associated with the EU region it just returns "Forbidden", not even a JSON formatted string! As a result all call fails.

I've coded up a proposed solution that I'm currently using and will raise a pull request on this repo and one for the docs.

[richardherbert]

Thanks for accepting the change, I'll raise a PR for the changes in the docs.