Add billing address to Identity

For registered (paid-up) Membership users, we should show their billing address as well as their normal (delivery) address, eg:

![image](https://cloud.githubusercontent.com/assets/394376/4704856/b36a3080-5875-11e4-96f0-a907c1f117a5.png)

I'm trying to get this to work but when I run the test without these fields I get a big compile error (I guess because they're not `Option`s?). But when submitting the form without them present everything is fine. Any ideas?