Added Examples for Stripe.js & Stripe Checkout

I've added a couple of examples for how to use go.stripe to create charges using Stripe.js (https://stripe.com/docs/stripe.js) and Stripe Checkout (https://stripe.com/docs/checkout).

The code is relatively simple, but it should help newcomers and promote the "safest" use rather than having the card details pass through their application.

Note that I've also updated a comment re: "USD only" to reflect Stripe's currently supported currencies with a link to their docs.

I've squashed the commits as well.