Adds Shopify-specific language to AM readme
eWay Rapid: Update supported countries and cards

Also adds invoice number support.

Closes #1396.
USA ePay Advanced: Fix test card expiration

Closes #1408.
Merge pull request #1494 from Shopify/update-readme

Adds Shopify-specific language to AM readme
BraintreeBlue: Improve AVS handling

BraintreeBlue gives back an AVS street response and an AVS zip response.
We now map those responses to the standard codes found in AVSResult.

The approach to do the mapping is a bit verbose but hopefully easy to
understand.  It's quite similar to how the Stripe adapter is handling
its AVS mapping.

All of the possible return values for the 2 avs results are listed here:

https://developers.braintreepayments.com/ios+ruby/reference/objects/transaction#transaction.avs-postal-code-response-code

The test uses Braintree's sandbox test values:

https://www.braintreepayments.com/docs/ruby/reference/sandbox#avs_postal_code_response_codes
PayPal: Allow specifying ButtonSource at init

In some environments one may want to set the ButtonSource on a per-
gateway basis; this allows overriding it in the @options has by
passing in a :button_source option.

If no :button_source is provided, application_id continues to be used
if available.

Closes #1495.
Payflow: Add fraud_review support

This adds support for the Response#fraud_review? flag in Payflow so
that merchants can be notified a "successful" transaction has been
flagged for further review before shipping.

Closes #1496.