Allow soft keyboard 'Return' key to dismiss keyboard on BTUIFormField

- Add a new optional delegate method `formFieldShouldReturn:` to `BTUIFormFieldDelegate` to allow handling the soft keyboard Return key
- Ask the form field to resign first responder when this fires in `BTUICardFormView`
- Implement `resignFirstResponder` to mirror the existing `becomeFirstResponder` of BTUIFormField
- Additionally, disable autocorrection for `BTUICardPostalCodeField`, to prevent iOS 8 text suggestions from appearing

This is needed particularly for `BTUICardPostalCodeField` (which isn't enabled for the default Braintree-Demo app), as it's the only standard card field that presents a keyboard with a Return/Done button, which before these changes does nothing.