Implement Stripe payments in twocloud package.

Fleshed out the stubbed Charge method in Stripe, and declared the
requisite errors for it. Charge now creates the charge in Stripe, and
updates the FundingSource record with the latest charge time.
Set Stripe API key from config.

Added a Stripe property to the configuration file and object, which is
used when creating persisters to set the Stripe key for the package.

I wish the Stripe package didn't rely on a global like this, but it
does, so it's kind of a hacky solution.