Keep the same order of arguments sent to the callback in _each when using $ or _

jQuery.each() calls the passed iterator giving (index, value), while _.each()
uses (value, index).

This fix makes both use (value, index).

This problem was introduced by me at 2364ad02e0cea232fca6770f25aa4c497c7f7740.

@rgrp I'm wondering how useful is it to support Underscore, as to use it, you'll also need https://github.com/wookiehangover/underscore.Deferred (right?).

If we move towards something like ckan.js, it will be good to avoid depending on anything at all.