Initial Paydollar integration

Moar work

Check with PayDollar API on notification

Change notification and return for PayDollar

Cleanup

Add tests for PayDollar

Clean up

Add remote test for PayDollar return

Tweaks
Update tests, fix bugs
Change routing of cancel/fail to simplify things
Remove obsolete URLs
Remove extraneous checking of currency code
Merge pull request #1038 from Shopify/paydollar_integration

Paydollar integration