Removed singleton instance

Prior to this PR, the `Stripe` instance was a singleton. This was required since `stripe.isInjected()` returned a boolean variable that was initialized when the script was successfully injected.

I realized that instead of memorizing a boolean variable as part of a singleton `Stripe` instance, I could verify if the Stripe.js script is injected using `typeof $wnd.Stripe !== "undefined"` in JSNI.

So, no more singleton instance :D