Implement Balanced Payments as a PhortunePaymentProvider

Summary:
Allows Balanced payment methods to be added. This works essentially the same way as Stripe, except everything is a little bit different.

Slightly more stuff could be shared, but I feel mostly good about this. I'll probably do a bit more cleanup next. Some of the error handling is messy, in particular.

Ref T2787.

Test Plan: Added Balanced and Stripe payment methods.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5765