Handle 1.0 errors and 1.1 errors
update 404 leetsoft.com links
update Gestpay url
Worldpay: Add support for external references

This allows referencing a transaction made in a different merchant
account by passing the :merchant_code option.

Also updates the DTD being used for Worldpay.

Closes #1081.
Fix unit tests hitting real API's
Sort fixtures file

And add a test that ensures it stays that way.
Merge pull request #1199 from alepore/gestpay_url

Update Gestpay URL in README.
Merge pull request #1198 from alepore/user_agent_url

Fix old leetsoft.com references