v4.0-beta

Documentation still needs to updated, however, all seems to be working for now. All models now inherit from AModel, which also contains some magic to create arrays out of our stacked models. In the near future, we should also filter Response types, but for now its fine.

From now, the API will be versioned according to the bol.com specification (so this version has been bumped to v4, because bol.com's api is version 4).