feat: follows $index change in ng-repeat

This works for the case I mentioned in #143.
Would someone review if it's the right way?
Since sorting the children of a fa-grid-layout (or an fa-scrollview, which requires similar functionality) is an expensive operation, I would hesitate to sort on every digest like this.  Especially when there are a lot of children or when there are multiple scrollviews/grid-layouts on the screen, this would take a performance toll pretty quickly. I did just respond to #143 though, hopefully with a viable fix for this.  Please let me know if that works for you.