Stripe: Include swipe_data if provided.

_Note: This field is not actually implemented by Stripe yet. We're waiting on it to get released (probably on Monday) so that this can be properly tested before being merged._

This change will include `swipe_data` in auth and purchase requests if it is provided.

Currently I'm using `.... if creditcard.respond_to?(:swipe_data)` (line 156), since I'm not 100% if we actually want to add a `swipe_data` parameter to the `credit_card` class. No other gateways make use of it. Thoughts on this?

Review from @odorcicd @jduff please