Litle: support wiredump_device

By specifying a wiredump device, such as:

  ActiveMerchant::Billing::LitleGateway.wiredump_device = File.open('/var/tmp/litle.log', 'w')

The device will now be properly passed to the LitleOnline gem for
logging.

The LitleOnline gem uses its own connection mechanism and the logging
hook will be available only in versions more recent than 8.15.

Testing Done: tested the integration with the Kill Bill Litle plugin
(see http://kill-bill.org).

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Litle: support paypage registrations

Allow to pass a paypageRegistrationId in the store() call, instead of a
credit card.

Testing Done: tested the integration with the Kill Bill Litle plugin
(see http://kill-bill.org).

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>

Closes #740.