Fix Diners Club validation. Fix null zip code.

Fixed null zip code entry in hashmap, that was breaking credit card validation.
Added a length-based validation for Diners Club cards.
Moved literals to constant fields for clarity.
Extended credit card prefixes according to wiki: http://en.wikipedia.org/wiki/Bank_card_number#Issuer_identification_number_.28IIN.29 
Updated .gitignore with IntelliJ files.