Add iOS 7 Compatibility

Hi Stripe!

I had to make a few changes to get this working in iOS 7. I thought you'd be interested in making this open to more devs who are not yet requiring iOS 8 from their users.
- `"Classes/**/*.xib"` was missing from resources.
- Moved minimum platform to 7.0, so you can support apps like us that don't require iOS 8.
- Removed `defined(STRIPE_ENABLE_APPLEPAY)` requirement which is set in our app but by isn't passed along to the cocoapod.
- Added bug fix for nil string