Better checkout.js stripe integration

[Marak]

There are two small issues ( so far ) with using the stripe.js checkout widget.

1. When using the slider on the /billing page. If a value is selected, then the stripe overlay is loaded by clicking on the pay button, then the value appears in the overlayed stripe form. If this overlay is then closed, and a new value is selected on the slider, then the pay button is clicked again...the value of the overlay is not updated. Unsure of the solution. The value inside the stripe iframe needs to be updated...or the entire element needs to be removed and added everytime it's closed / loaded.
2. If data-email is provided to the stripe checkout.js widget, then the email field will be pre-filled. This is good, but the input box becomes un-editable, which is not good. I'd like to see the email field pre-filled, but still be editable. Possible solution might involve contacting Stripe / making a change to the checkout.js file itself / or just finding a configuration variable we can set.

[Marak]

The stripe integration has been improved a bit.

We still aren't able to bind default data-email to the widget.

Leaving open until we can sort out that last detail.