Adding 2Checkout payment method

Adding 2Checkout Payment Method

This adds support for 2Checkout for both recurring and non recurring invoices. Sellers can also choose to use either direct checkout (opens in an overlay) or dynamic checkout (redirects to 2Checkout). I have maintained this code for a while at https://github.com/craigchristenson/2checkout-wp-invoice and it is currently being used in production by 2Checkout sellers.

I just setup a fresh install of wordpress and tested against the development branch to insure that everything is still working as it should.

Let me know if you need anything or would like any changes and thanks for letting me know that you moved to Github. Feel free to throw any merchant (user) questions at me. I am happy to continue maintaining.