Moneris AVS suppport.
Changes based on feedback.
AVS enabled all the time.
Remove obsolete comment.
Revert to original fixtures since test store choice does not affect AVS.
Remove extra fixture modification.
Merge pull request #1267 from Shopify/moneris_avs

Moneris AVS suppport.
Add Commercegate gateway

http://www.commercegate.com/

Close #1027.
Wirecard: Fix "amex" references

Close #1263.
HPS: Do not pass empty elements

In particular, do not attempt to send any CardHolder element unless
there is a first name or last name available at a minimum.

Also do not send an empty CVV element.

Closes #1269.
Paymill: Add more supported card types

See #1264.
Paymill: Add source

See #1264.
Paymill: Add description for preauthorizations

See #1264.
Paymill: Add remote tests using tokens

Closes #1264.
HPS: Add additional credential options

Close #1271.
Update 1stPayGateway.Net gateway

This replaces the now broken 1stPayGateway implementation with support
for their modern API and all five major transaction types.

Closes #1272.
Payflow: Add verbosity option

Closes #1275.
Allow ignoring any MultiResponse sub-response

This enables use (for instance) to void an authorize when verifying a
card without a failure of the void causing the verify to fail.
Stripe: Add ability to verify a card

For Stripe, a verification is an `authorization` followed by a `void` of
that authorization.

The minimum authorization Stripe allows is 50 cents so we use that.

Relevant docs:
https://stripe.com/docs/api#create_charge

We're now taking advantage of the ability of a MultiResponse to ignore
the result of some of the responses.  In this case, we don't want the
`verify` to fail if the `void` fails.  The important part is the
`authorize`.

Closes #1279.
Paypal: Add ability to verify a card

Paypal supports zero dollar authorizations, but only for mastercard and
visa.  Relevant docs:

https://developer.paypal.com/docs/classic/paypal-payments-pro/integration-guide/WPDPGettingStarted/#cardverifications

To verify a card, we do a zero dollar `auth` if it's mastercard or visa.
Otherwise, we do a $1 `auth` followed by a `void`.

We take advantage of the new ability in MultiResponse to ignore the
results of parts of the response.  In this case, if the void fails, we
don't care.

Closes #1280.