Restrict visa valid digit lengths to 13/16

As per http://en.wikipedia.org/wiki/Bank_card_number#Issuer_identification_number_.28IIN.29 visa numbers are only 13/16 digits long. This prevents numbers like `4111 1111 1111 22` from incorrectly passing `validateCardNumber`
Merge pull request #79 from ajlai/patch-1

Restrict visa valid digit lengths to 13/16