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
