Addition of CCs/Bank accounts even if form elements not present on page load.

If the form elements for adding bank accounts or credit cards are added to the DOM after initial page load (ie AJAX) the submit button is not present to attach .click() and trigger the calls to balanced.

Fixed by moving each file into their own function, called by a jQuery .on handler on the submit button class which must exist.

Allows the addition of either bank accounts/credit cards even if the form is added after initial page load.
@vadimt The other issue that this solves: 

It isn't so simple to not include:

```ruby
#- content_for :javascript do
= javascript_include_tag "https://js.balancedpayments.com/v1/balanced.js"
= javascript_include_tag "transactionable_add_credit_card.js"
```

into the head in an AJAX'ed partial and expect the Balanced payment js to be loaded by the time: 

```ruby
  = javascript_tag do
    $(document).ready(function() {
    var marketplaceUri = "#{Balanced::Marketplace.my_marketplace.uri}";
    balanced.init(marketplaceUri);
    });
```

will execute. I ended up having to setTimeout this script ^ due to the included Balanced Payment js loading asynchronously from the rest of the view. 

It is better to include these javascripts on the payment page initial load, and attach these event handlers to body that trigger on a submit click for the CCs/Bank accounts. This way, the README also still makes sense in most cases.