Retrieve payment method from the payment source

[aldesantis]

Currently, the payment method needs to be set via the configuration.

This makes it harder to allow users to subscribe through different payment methods, since it requires monkey-patching the Checkout#active_card method.

Instead, we should retrieve the payment method directly from the payment source that's being charged, which should be easy enough via active_card.payment_method.

This spares the user from having to set the gateway manually and lays the groundwork for #116.

[aldesantis]

Closed via #121.