Added implementation for multiple preset configurations.

I added the functionality to configure multiple presets. This will give you the possibility to make more detailed configurations. I tested it with two nodes and it works, even if both are named "default". If the serialization node is missing completely, Sitecore will throw a `NullReferenceException` when you try to save an item. But as I read the code, this would be the same behavior as before the change. But seriously, who would not specify a serialization node when using Unicorn ;)

If you'd like, I could update the serialization.config file in the documentation folder to show the possiblity. Just let me know.