Extract arrays.delete() to core/utils

I've cloned the array using `slice()` without argument (which is what underscore's` _.clone()` does for arrays: http://underscorejs.org/docs/underscore.html#section-88)
Because of:

```javascript
var array = JSON.parse(localStorage[key] || '[]');
```

Without the conditional it sets the localStorage key to an empty array, is that what we want?