Uses Luhn algorithm to check validity of credit card numbers.

List of methods:

• validateCred (array): accepts an array with one credit card numbers, returns boolean. Checks validity of the credit card number by using Luhn algorithm: every second array item doubled, starting from the right; if the result has double digits, decrease it by 9 (or add digits together, same thing: sum is never higher than 18). If result modulo 10 is equal to 0, the number is valid
• findInvalidCards (nestedArray): accepts a nested array of credit card numbers, returns a nested array of invalid credit card numbers
• idInvalidCardCompanies (nestedArray): accepts a nested array of credit card numbers, returns an array of unique credit card company names. Intended for invalid card number nested arrays.