Use Stripe return values to check fees paid
Current behavior: 'fetchProject' guesses how much money a project has received by summing up how much money the mechanism believes patrons have donated. But Stripe isn't actually consulted.
While the numbers should always add up, I'd rather that we check it right away, and blow up if it doesn't compute! 'makePayments' should inspect the BalanceTransaction object (within the Charge object) to confirm charge amounts, fees paid, etc.