Execute callback on operation finish

This adds a callback parameter to all the modify operations.

Additionally it removes the callback parameter of the find methods since it wasn't used
anyway.

It might be worth thinking about reintroducing the callback parameter to the find methods and always executing them on any change i.e. give them via a parameter to the observe method and execute them before the angular scope apply().
On the other hand it's possible to `$watch()` the changes on the object anyway with the fixes from my other pull requests, so that might not be needed after all.