Autovivification for collections.

This is a simple implementation to (optional) [autovivification](http://en.wikipedia.org/wiki/Autovivification) for collections.

I'm not sure if you want to implement this behaviour to Backbone.js but I needed it in a project so here is the idea. If you are interested we can discuss about the implementation on a new issue.

I'm sorry if I missed any code/naming standard in this project.