Add support for Authorize.net in CA and UK

##### Changes

Sends the buyer's currency to Authorize.net during authorize and purchase requests. Also adds 'CA' and 'UK' to the supported_countries list.

**Note:** I originally had a remote test included which would send test requests using both the CAN and GBP currencies. However this would always fail with the message: 
*"The supplied currency code is either invalid, not supported, not allowed for this merchant or doesn't have an exchange rate."*
Looking through their documentation, it states the the merchant account must have certain payment processors enabled to handle these different currencies. However after getting in contact with Auth.net, they have said that their test environment only works with USD since it does not have the required payment processors available. It seems we will not be able to properly test this until it is using live servers...

The changes are straight foreword and all existing tests which use USD are still passing as expected so it seems safe enough - but I'm not completely sure what we want to do with this one.

##### Reviewers
@jduff 
@Soleone