Add WePay as a one-time payment provider

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