Add id to refunds

This PR adds an "id" field to each refund object in the response to a refund call (see https://stripe.com/docs/api#refunds).  

It builds on two other PRs which are unmerged right now from @sman591 (https://github.com/rebelidealist/stripe-ruby-mock/pull/105 and https://github.com/rebelidealist/stripe-ruby-mock/pull/106).