Update postal code logic

[davidme-stripe]

Summary

• Request postal codes by default in most situations. This is not a breaking change to our public API, though it may cause trouble if folks are messing with our internals.
• When STPBillingAddressFieldsZip is selected, STPAddCardViewController will now embed the postal code collection into the STPPaymentCardTextField instead of splitting it out into a separate row.
• Postal codes will now be validated for non-US markets. We're assuming that they're always uppercase, and include only alphanumeric characters, dashes, and spaces. These rules will also apply in the US, as we can't be 100% sure that a US region user will have a US ZIP code. Shipping/Billing address entry forms will still validate and enforce US-style ZIP codes for US addresses.
• All countries will use the NumbersAndPunctuation keyboard.
• STPBillingAddressFieldsZip has been deprecated in favor of STPBillingAddressFieldsPostalCode, which is identical. I'm not sure whether this is worth the churn, but it didn't seem appropriate to use "ZIP" now that it's the default option globally.

Motivation

We're now recommending that all users in the US collect postal codes.

Testing

• On a device, switching between a few regions and exercising different STPBillingAddressFields settings.
• I've also added a new set of end-to-end tests using a French device in Belize, covering both the "different language" and "doesn't use postal codes" case.

[csabol-stripe]

yay UI tests!

Looks good with a couple questions inline

We should probably add some text to MIGRATING.md describing how the default behavior has changed and why you probably don't want to revert the change but here's how you would if you really want to

[davidme-stripe]

@csabol-stripe How's this? https://github.com/stripe/stripe-ios/blob/davidme/update-zip-logic/MIGRATING.md#migrating-from-versions--1900