Correct check for global `exports` for commonJS and Browserify

```coffee
typeof exports is not 'undefined'
```

Translates into:

```js
(typeof exports === !'undefined')
```

Definitely not what we want.

Instead, using [`exports?`](http://stackoverflow.com/a/10238211/790169) which will properly translate to... 

```js
(typeof exports !== 'undefined' && exports !== null)
```