Added basic AMD support

Maybe not necessary, but here it is if you'd like.

For example:

```html
<!-- index.html -->
<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Test</title>
  </head>
  <body>
    <script src="require.js"></script>
    <script src="braintree-1.2.0.js"></script>
    <script src="app.js"></script>
  </body>
</html>
```

``` javascript
// app.js
requirejs(['braintree'], function (Braintree) {
  Braintree.create("########");
});
```