Allow Stripe::Token to accept customer id instead of card

When using [Stripe Connect's Shared Customers](https://stripe.com/docs/api#create_card_token), the [Create Token API](https://stripe.com/docs/api#create_card_token) is used to generate a token in place of a customer's card token.  Many times, including in Stripe's example for Shared Customers, you'll only need to pass in the `customer_id`.

This change makes it possible to pass in just the customer to the `create_token` method.