Pre-determining the current breakpoint on configuration

Hi Ryan, in my current project I'm binding quite a lot of logic to the crossover-event and realized, that after the page being loaded, the crossover-event is being fired when resizing the window for some px (but not actually crossing a breakpoint). The problem resides in the `Elemset.reset()` method

```js
// tempIndex is the index of the breakpoint
// while this.i === 0 initially
if (tempIndex !== this.i) { /* ... */ }
```

So I added some logic to determine `this.i` initially inside the configure-method. Maybe I'm doing something wrong here and this is rubbish, but please check my changes. Cheers, Alex