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.
That sounds really cool, but I haven't been able to figure out a good way to make that work. If you go simple and just say "confirm the test credit card number doesn't appear in the scrubbed remote transcript", you get coverage on the credit card but you still need to test the other things that this gateway does. I suppose it wouldn't be that bad to add, but I haven't figured out a great way to do it that isn't duplicating what you're doing in the `scrub` method in the remote tests when you go to make sure you scrubbed ok. I'm open to suggestions though!