Gateway transcript scrubbing

This is a first pass at adding transcript scrubbing to ActiveMerchant gateways - here's what I've done:

1) Implemented scrubbing for Stripe
2) Implemented scrubbing for PayPal
3) Added `Gateway#supports_scrubbing?` so you can check which gateways have implemented scrubbers.
4) Added a scrubbers stub to the Gateway template, as well as a remote test that will write out the transcript for a purchase call and then fail (so that you can fill in scrubbers for the gateway you're implementing).

This PR doesn't change access to the transcripts, because ActiveMerchant isn't actually capturing them (they are only captured in the one remote test, in order to be used to write your scrubbing code) - if you want to capture and then scrub them, you have to do that part yourself.
I like having this only in the tests; with great power comes great responsibility, and we probably don't want to make it *too* easy to capture transcripts.