Added the option to use an animated collection view layout update, and t...

I found your repository [on Stack Overflow](http://stackoverflow.com/a/17518226/903544) and decided to add a two features.
The first one will fit the cells while zooming by maintaining the spacing between them. The second one will use the built-in animation to change the collection view layout, instead of directly invalidating it.
Both can be enabled with a BOOL parameter in `ViewController.m`.