Initial order of array on fetch collections

This patch adds an index field with the order they had into the initial array.

It allow us to do a sort on the models by that field. i.e.

Job.fetch()
Job.all().sort(Job.sortByIndex)

where sortByIndex would be:
  sortByIndex: (a, b) ->
    if (a.index > b.index) then 1 else -1

It prevents the problem of the ordered hashes for Chrome and some mobile browsers.