Fixed confusing use of invert boolean

This line was bugging me slightly, firstly it may be a typo from something that was regressed, and secondly JSHint didn't like it:

```
[L6:C21] W018: Confusing use of '!'.
    var supported = !(typeof storage === 'undefined');
```

This fixes any breaks if running the source through JSHint, and removes unnecessary wrapping of that operator.