Add Cashnet gateway

http://www.higherone.com/

Closes #1166.
Conekta: Standardize address options

Closes #1154.
Wirecard: Add support for reference purchases

Closes #1192.
Eway Rapid: Update country list

I don't see anywhere on the eWAY website to suggest that Rapid has any
availability difference from the straight up eWAY gateway.  Added New
Zealand and the UK to the list.

Closes #1194.
Send along proper version header for Balanced gateway.

Closes #1196.
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
Balanced: Voiding a capture is not allowed

Balanced only allows authorizations to be voided.  Previously, if
someone attempted to void a capture, it would raise a TypeError since it
as trying to append a string to nil.

Now we return a failed Response.

Closes #1201.
Add HPS gateway (Heartland Payment Systems)

http://developer.heartlandpaymentsystems.com/SecureSubmit/

Closes #1069.
Cashnet: Add a few remote tests

We now test that failures are working as expected.

Closes #1203.
Balanced: Fix handling of 500 errors
Remove billing fields from Universal
Merge pull request #1210 from Shopify/remove_univeral_billing

Remove billing fields from Universal Offsite
Balanced: Stop creating a customer on each call