Add support for Carthage

[alejandro-isaza]

It should be as easy as creating a project with a shared Framework target. I could send a PR.

See https://github.com/Carthage/Carthage

[jflinter]

Hey @Aleph7,
This code has moved to https://github.com/stripe/stripe-ios, which now has a class called STPPaymentCardTextField with similar behavior to PTKView. Our iOS SDK supports Carthage installation, so you should be covered there (just add github "stripe/stripe-ios" to your Cartfile). I'd encourage you to migrate (it should take just a few minutes, and we've written a brief guide at https://github.com/stripe/paymentkit#migration). Of course, if that doesn't work for you, or you have any trouble migrating, reply here and I'll be happy to help.
Cheers!