Add main field and publish on npm

This will allow usage like

```js
var createButton = require("PayPalJSButtons").apps.ButtonFactory.create

var button = createButton(window.paypalMerchantId, {
    name: "my-app",
    amount: "80.00",
    currency: "USD",
    recurrence: "1",
    period: "M",
    env: "sandbox",
}, "subscribe")
```