Change function scope

With this tiny refactor the fwd/rev functions are now available on the window object and can be tapped into from presenter remotes. Here's my simple use case after the change:

```javascript
<script>
    document.onkeyup = function(e) {
        // keycodes emitted by Satechi presenter remote
        if (e.which === 40) fwd(); // left button
        if (e.which === 38) rev(); // left button
        // if (e.which === 33) { /* up button, do nothing */ }
        // if (e.which === 34) { /* down button, do nothing */ }
    };
</script>
```