Should auto-bind to cardType on gold-cc-input

[tigerhawkvok]

Why do devs have to touch this at all? It's already a custom element, why not have the smarts built in?

Proposed behaviour:

1. Give it a for attribute, which takes a selector for a <gold-cc-input> element.
2. If that's not there, and if the parent <form>, <iron-form>, or non-<form> first parent also has a single <gold-cc-input> as a child, then bind to that
3. Otherwise devs are left with the current behaviour

It should, practically, also solve #18 (as it's very rare for a user to enter the CVC before the card number).

[notwaldorf]

I don't think tying them together needs to be that complicated. Since cardType is a notify property, you can just do something like:

<gold-cc-input auto-validate card-type="{{cardType}}"></gold-cc-input>
<gold-cc-cvc-input amex="[[_computeAmex(cardType)]]"></gold-cc-cvc-input>

I realize that's a bit annoying, however, so I'll change gold cc-cvc-input to have a cardType property instead of amex, so that you can just write:

<gold-cc-input auto-validate card-type="{{cardType}}"></gold-cc-input>
<gold-cc-cvc-input card-type="{{cardType}}"></gold-cc-cvc-input>