Element.dataset is not supported in IE;

<recreated from https://github.com/joemccann/openwebsxsw/pull/2; Original change made on production file;>

Currently there is no way to make a invoice payment using Internet Explorer due to undefined being returned for the stripeConfig properties. Clicking the button does not instantiate the Stripe modal dialog.

IE shows an error of "Unable to get property 'key' of undefined or null reference 
 openwebsxsw.js, line 143 character 5"

IE does not currently support the dataset mapping. Rather than add a polyfill, I've fallen back to using the getAttribute(String) method.