Summary:
Ref T2787.
I //think// we could also use WePay as a recurring payment provider, but this is somewhat messy (OAuth + requires account) -- basically it's "add a WePay account" instead of "add a credit card".
The WePay checkout workflow is a bit upsell-y but basically reasonable.
I like that their API just has a `request($method, $params)` method instead of 30,000 lines of methods for each request type. I did hit one bug; I'll send a pull for that.
Test Plan: Got as far as the charge callback in testing; the rest isn't implemented for any provider yet.
Reviewers: btrahan, vrana, chad
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2787
Differential Revision: https://secure.phabricator.com/D5982
