Add test case for serialization of custom objects.

Twitter Chill is a great project but it's sparse in documentation.  I want to serialize my own classes using the serialization routines Twitter has for base types.  In particular, I want a high-level abstraction so for my custom class `Foo` I can write `Serializer(foo)` and use `Serializer.invert(bytes)` to get a deserialized foo.

I wrote a test case that demonstrates how I would do this.  It's easy to write Bijections between case classes and tuples and register them with a custom KryoInjection class.  Please merge this if you think this is the "proper" way to do this.  Otherwise, let me know what I'm doing wrong and how I can do it right!