This pull request fixes some lint warnings.

JSHint, UglifyJs and other tools capable of linting complaining about some unused, but declared variables. This pull request gets rid of these.

Note: Fixing the `exports` issue that way allows one to use `lib/b64.js` natively in browsers, e.g.:
```html
<script src="https://raw.githubusercontent.com/beatgammit/base64-js/master/lib/b64.js"></script>
<script>
  var base64string = base64js.fromByteArray(...);
  var bytes = base64js.toByteArray(...);
</script>
```