Add slice utility

Relatively compliant slice -> array
To bypass the `for()` there are two things you could do.

First I'd check to see if `this.start < this.end || (this.start + (end - start)) <= this.length`. In that case you can use a simple `Array#slice()`.

In the other case it could be simplified down to something like so:
```javascript
var result = this.slice(this.start, this.length - this.start);
result.push.call(result, this.slice(0, (end - start) - (this.length - this.start)));
```

That's just off the cuff, so not sure if it's fully working, but think it gets the idea across.