Add document_back property to STPVerificationParams

[rance-attack]

Summary

Add document_back property

Motivation

Can see this property in go project (https://github.com/stripe/stripe-go/blob/master/account.go#L211)
Hope iOS SDK can also have it

[csabol-stripe]

Looks good to me. Thanks for the contribution!

[danj-stripe]

@rance-attack Yeah, we appreciate the contribution, and your follow up commit w/documentation that fixed the build!

FYI: we do have additionalAPIParameters on all of the params objects, so you can immediately start using properties (like document_back) that the Stripe API supports but that the SDK doesn't yet have:

stripe-ios/Stripe/PublicHeaders/STPFormEncodable.h

Lines 28 to 35 in 7116259

	/**
	You can use this property to add additional fields to an API request that are not explicitly defined by the object's interface. This can be useful when using beta features that haven't been added to the Stripe SDK yet. For example, if the /v1/tokens API began to accept a beta field called "test_field", you might do the following:
	STPCardParams *cardParams = [STPCardParams new];
	// add card values
	cardParams.additionalAPIParameters = @{@"test_field": @"example_value"};
	[[STPAPIClient sharedClient] createTokenWithCard:cardParams completion:...];
	*/
	@property (nonatomic, readwrite, copy) NSDictionary *additionalAPIParameters;

It won't have the type-safety that the property declaration does, so we definitely want to fix the SDK like you've done here, but it would allow you to use the SDK without forking it.