More testing of error behaviours
Some testing of the 'SendMail()' method
Testing that SendMessage() returns ID from response
Renaming test to be the real method name (sendMessage, not sendMail)
Testing that appropriate error is thrown when unexpected response from mailgun
Testing that attachments get sent correctly
Testing basic optional arguments are sent to MailGun when specified
Ability to send all documented optional variables of the SendMessage() method through the API