Added CallbackObject property for callbacks on objects other than its own

I was using your library in a project and noticed the necessity for Ani objects to be able to invoke callback methods on objects other than their own. That way, an Ani can notify other objects e.g. of its end. It improves the usability of the library in more complex projects greatly.

I also created new Ani.to and Ani.from signatures that include the CallbackObject as a parameter. In case the parameter combinations don't fit for every need, there are of course also getter and setter methods.