Fixed several JS errors

[jmauzyk]

I encountered several JS errors with the Drop-in UI that I've addressed here.

• I realized I could resolve #47 without passing a gateway handle. This is addressed in lines 24–29. If no drop-in UI element is found in the form, the function returns null to prevent trying to access a value on a null element.
• A couple instances of querySelector('button[type="submit"]') could result in errors if an input element was used to submit the form instead of a button. Updated the selectors to address this.
• On line 127, addEventListener() is not a direct replacement for jQuery on(), and because the parameters weren't formatted correctly, the event wasn't triggering. Because addEventListener() doesn't pass data the way on() does, it's necessary to pass the data another way. One way is using currying, which I implemented on line 127, seen in formSubmit().
• Replaced a remaining call to jQuery val() with .value on line 168.

[jmauzyk]

@samuelbirch I see you resolved some of these issues with your latest commit, thanks for that. However, the call to addEventListener() remains the same as before. As written, the form does not generate a nonce via JS on submit, but instead directly POSTs the payment form in the browser. This then redirects to the payment page with the error Cannot determine payment method. Are you still working on a solution for this?

[samuelbirch]

New version released with fixes for this.