Stripe: Support multiple cards on account

Add support for multiple payment methods per customer.
The `store' call now adds a payment method if a customer
is specified (it used to clobber the default card instead).
You can specify :set_default => true to set the newly created
card as default.

The `update' call will add the new card and set it as default.

A new `update_customer' call has been added to update a customer's
metadata.

This commit also features a small refactoring in Stripe remote tests
to easily swap the default currency used.
Stripe: Add card_id parameter to unstore call

Specifying a card_id will now delete the card only, without
deleting the customer.

Closes #895.