Sort order of properties with the same index (#94)

Issue #94 + PR #104

The common problem appears to be with declarations that have same properties.
For example:

```css
a {
    width: 0;
    width: 100%;
}
```

`Array.prototype.sort` gets confused about which declaration should appear first since they have the same group index and the same property index.
So I've decided to save node's index number as one of extended node's properties and then use it inside `sorted.sort()` as a helper.
It does the trick.