Rough and ready prototype of custom serialization

[jskeet]

This is to demonstrate one possible approach to #2534.

[jskeet]

Thanks for the comments. I'm pretty sure we want to be able to separate converter from the model itself. (All serialization approaches I've seen which are based on the model implementing the interface end up being painful with respect to immutability, at the very least.)

I think we need to discuss requirements and implementation options around the MailAddress and "shared model" scenarios. (Is the latter even a scenario which is wanted, for example?)

[jskeet]

Closing this now as the real implementation is on the way.