Expose the list of available card types

Hi,

I have a DOM that looks like this, with the card type class on the container:
```html
<div id="credit_card_number_container">
  <input type="text" id="credit_card_number" />
  <span class="placeholder"></span>
</div>
```

And I was trying to have a stateless event like this: (this is pseudo-code)
```javascript
$ccn.on('keypress change', function (ev) {
  if (card.isValid()) {
    $ccn_container.removeClass(all_card_types).addClass(card.cardType());
  }
});
```

Unfortunately, the list of card types wasn't available, so I had to hard code the list of card types.

Of course, the quick and dirty way to what I want to achieve would be something like: (pseudo code again)
```javascript
var previous_cardtype = '';
$ccn.on('keypress change', function (ev) {
  if (card.isValid()) {
    $ccn_container.removeClass(previous_cardtype).addClass(card.cardType());
    previous_cardtype = card.cardType();
  } else {
    $ccn_container.removeClass(previous_cardtype);
    previous_cardtype = '';
  }
});
``` 

But I do think that stateless code is more robust.

What do you think?