Add Ludwig integration

Hi!
We would like to ensure that all Commerce 2.x payment gateways have Ludwig integration, to ensure that they can be installed without Composer.
You can find out more about Ludwig from this blog post: https://drupalcommerce.org/blog/49669/installing-commerce-2x-without-com...
And you can find a sample Ludwig integration here: http://cgit.drupalcode.org/commerce_ingenico/commit/?id=364f5ea

Notes:

- If your SDK has a dependency of its own in its composer.json, you must list that in the ludwig.json file as well

- You do not need to list dependencies that are included in Drupal core (such as Symfony)

If you have any doubts, you can post a patch to this issue first, and then someone from the Commerce Guys team will review.

Thank you!