Add CreditCards number, cvv fields as updateable

Since Braintree allows for these fields to be part of a call to
update credit cards, I don't see a reason to leave them out.

This way, we're able to simulate a one-to-one Customer-CreditCard
relation without diving into complex delete-and-create logic.
Merge pull request #4 from wazeHQ/master

Add CreditCards number, cvv fields as updateable