Alternative payment backends

[mutantmonkey]

For various reasons, some people may be unable or unwilling to use Stripe. It would be nice to support alternative payment backends such as Authorize.net, Braintree, Square, or PayPal.

[echarlie]

One concern for this: to do things like #13, #9, #35 (?) and others we would need some sort of shim to speak to all of these different backends. This is a good design, so I think it will be fine in the long run, but we'll need to set up some automated testing to do this.