Tried to throw MissingRequiredMIMEParameters which didn't exist in current namespace or added via use. Connection exceptions was included by MissingRequiredMIMEParameters is in messages exceptions so changed the added exception name to messages.
Exception message constant didn't exist. So added it.
Created a Mocks directory to seperate mocks from classes.
I moved Mailgun\Tests\MailGunTest to Mailgun\Test\Mocks\Mailgun. As it's not a
test it's a mock. So shouldn't be suffixed with Test.
I mocked Mailgun\Tests\Connection\TestBroker to Mailgun\Tests\Mocks\Connection\TestBroker as it's a mock and should be with other mocks.
