Add basic payment providers to Phortune

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