Experimenting with element/its type relationship

[laughedelic]

The idea here:

• ElementType type is just a holder for the unique label and a factory for creating TypedElements
• TypedElement extends it. It holds the raw value and add all those graph API methods.

It comes from the idea that in the end we want just the User type for working with its values and the VertexType value user to refer to it as Users factory and the properties holder.

In the end I'm not sure this idea work well. It does simplify some things, but doesn't improve the things much and you still need to write some boilerplate when defining User: self(), fromRaw() and the constructor.

[laughedelic]

Closing this. It was an experiment. See #62 for a better try.