active_merchant: make void smarter

CyberSource has a different method for reversing an auth.
It looks like it's the only one though.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
active_merchant: pass the transaction external key as the order id

It is friendlier to pass external keys to the gateway as they are known
to the API user (whereas the kb_payment_transaction_id is internal to Kill Bill).

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
active_merchant: add before_gateway / after_gateway hooks

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
spec: fix tests failures

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>