Add possibility to use not only Long for id.

Unicorn classes previously typed with [something <: BaseId] now are typed as [something <: MappedToBase].
Using MappedToBase instead MappedTo[T] allows to omit type T of underlying value and therefore to keep methods signature unmodified.
 The MappedToBase class is a superclass of BaseId so all previously written code should work.
 Add test checking correct work of UUID id