Add factory func to schema

[YuukanOO]

Sometimes, you may want to construct a TData by using a custom factory method when reading triples back to JS object. For now, the schema only gives back an object without prototype.

By giving the client the opportunity to provide a custom factory method, we enable a lot of scenario when a user want to retrieve object of a specific prototype.

I was first thinking of using the Object.setPrototypeOf() in the schema.read method but since this is not recommended, providing a custom func makes more sense.