readme doesn't suggest code that will pass unit testing

```
var element = document.querySelector('.class-name');
var styles = styler(element).get(['height', 'width']); 
```

Should read 
```
var element = document.querySelector('.class-name');
if (element) {
    var styles = styler(element).get(['height', 'width']); 
}
```
To prevent errors when running unit tests. 