Custom JSON encoder

[janrito]

It would be nice if we could specify a custom encoder (and decoder) for types that will need extra deserialisation – e.g., sets, enums or sparse matrices – or even project-specific types

[rsyring]

I would also like to see this possibility. I suggest (#612) using a method on Database so it can be easily subclassed. Another option would be to pass in callables to Database() for json serialization and deserialization.