Add handler for get /v1/customers/:id/cards

This adds support for this endpoint : https://stripe.com/docs/api/ruby#list_cards

I had to add this weird count thing in the handler otherwise the card count would always be 1.
Let me know if there is a better way to achieve this, I would be more than happy to update the PR.

I used this PR https://github.com/rebelidealist/stripe-ruby-mock/pull/77 as an inspiration.