Refacted card validation + txt field delegate

[Haud]

This resolves issues #7 and #8 proposed by @MichaelCampbell.

For #7

• The delegate is now called on every single input along with a card and validation object; the delegate no longer accepts nil inputs for those parameters. Because of this, the card object passed in the delegate function is not guaranteed to be valid; the validation parameter will dictate if the card is valid or not and which parts are not valid. It is up to the receiver to check the input.
• Refactored validation and moved it all to CardType extensions as the existing validation classes were too interdependent and, in some cases, redundant

For #8:

• New method: toString() on CardValidationResult that returns an array of all strings.
• description now maps the results of toString() to a multi line string with no { }; any other formats should now use toString with their own mapping.

Please look through and make sure these changes are the desired functionality.

@DannyVancura

[DannyVancura]

Looks great! Couldn't find anything wrong with that implementation.

[MichaelCampbell]

👍 works well for me too. Thanks for simplifying the month and year extraction too