Implement Model functionality

Models in Guacamole should not inherit from another class but get their functionality by including a `Guacomale::Model` module (It should be ensured, that attribute methods could be still overridden by the implementing model).

Timestamp attributes should be automatically added to the models by including the `Guacomale::Model`. For now we won't allow opting out of those columns. Additionally a `key` and a `rev` attribute must be made available. The `id` field from ArangoDB should **not** be exposed to the `Guacomale::Model`.