Add in the ability to create payments for a given loan using a JSON API call. You should store the payment date and amount. Expose the outstanding balance for a given loan in the JSON vended for LoansController#show and LoansController#index. The outstanding balance should be calculated as the funded_amount minus all of the payment amounts.
A payment should not be able to be created that exceeds the outstanding balance of a loan. You should return validation errors if a payment can not be created. Expose endpoints for seeing all payments for a given loan as well as seeing an individual payment.