Moneris: Add :avs_enabled option
Wirecard: Improve error handling

If Wirecard is having a really bad day it is possible for their API to
return error responses that are not contained within the W_JOB element.

This adds a new method to correctly parse these Wirecard
having-a-bad-day responses, and also pluck out the ErrorCode as well.

Closes #1315.
Merge pull request #1317 from Shopify/moneris_avs_flag

Add :avs_enabled option for Moneris