Summary: Abstract out the Stripiness of payment providers so we can add a Test provider (and maybe MtGox / Balanced / Paypal / etc). Ref T2787.
Test Plan: Ran unit tests.
Reviewers: btrahan, chad
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2787
Differential Revision: https://secure.phabricator.com/D5752
