"Collection: add into sorted position" + index is passed in 'add' event

[lawrencepit]

When a Collection has a +comparator+ defined, then when adding an object the +add+ event handler needs to know at which position the object was inserted into. This patch passes the +index+ to the +add+ event handler.

[wookiehangover]

The model's index is readily accessible in an add event handler via indexOf:

bind('add', function( model, prop ){
    console.log( model.collection.indexOf(model) )
});

[snoe]

+1 for the patch

The index is commonly needed so that you can insert the new info in the correct place

$("list li").get(index).after(view.el)

In a large sorted collection model.collection.indexOf(model) could be expensive. Backbone is already calculating this index and then throwing it away.

This is good for the same reason that Array.forEach passes the item and its index to the handler.

[wookiehangover]

this is probably a good candidate to consider along side a fix for #689

[jashkenas]

Backbone is no longer calculating the index on a per-model basis. For the time being, if you need it, you can look it up.