Add on change callback

It is sometimes useful to monitor changes to an angularFireCollection as items are added and removed. This mod adds an optional callback parameter to angularFireCollection. The callback will be called whenever any item is added to, removed from or updated in the collection.

An alternative to adding a third parameter is to allow the second parameter to be an options hash e.g.
    { initialCb: init_callback, itemChangeCb: item_callback }