Elavon: Upgrade gateway integration [WIP]

[leila-alderman]

This is the start of the work necessary to upgrade the Elavon gateway integration to move from the deprecated process.do endpoint to the more current XML endpoint processxml.do.

This WIP commit includes the major updates to the gateway structure to use XML and to achieve a successful remote purchase request.

Many of the tests still need to be updated, including updating all of the stubbed responses in the unit tests.

CE-704

ruby -Itest test/remote/gateways/remote_elavon_test.rb -n test_successful_purchase

1 tests, 5 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed

[curiousepic]

Marking this "of interest" before a cleanup of stale PRs