Revert "Revert "Authorize.Net: Switch to the XML API""

This reverts commit 0b8a0de2e2d547b96e0d1aac63d5ace8baf0e9ff.
Move customerIP to be below customer block in Authorize.net post_data XML
Merge pull request #1367 from Shopify/unrevert_authorize_net

Unrevert Authorize.net