Add NotBlank requirement to credit card number

Funny how we missed this, but except for the cardholder name, none of the credit card fields were required..
Found this out the hard way in production :)

Also, I'm adding a leading zero to expiry month, in keeping with other checkout forms out there.
Merge pull request #866 from Richtermeister/fix-credit-card-validation

Add NotBlank requirement to credit card number