Stripe integration tests that actually hit the Stripe API

Using our account's test key will allow us to really communicate with Stripe and get real responses (albeit with imaginary money). We should use this to write tests about our Stripe assumptions.

• Are we calculating the fee correctly?
• Do the API commands we use exist and respond as we expect them to?

We don't want to spam the crap out of Stripe's servers, so access to the test key (and the ability to run integration tests) will be limited.