Changed how stripe-form detects values

Rather than depend upon the `scope` to be named with specific values, `stripeForm` can now pick off values by the naming of form elements.

I.e. Rather than depending upon `ng-model="number"`, if there is a field in the form with the `name="number"` it will work. 

```
<input type="text" 
name="number" class="input-block-level" ng-model="payment.number" payments-validate="card" payments-format="card" payments-type-model="type" ng-class="myForm.number.$card.type"/>
```

This is a much more Angular-ish approach.