Fix the bug in `ModelClass.extend` where the `base` option used
in the new class was inherited from ModelClass. As a result
the extended model was incorrectly based on ModelClass's parent.
Modify `modelBuilder.define` to normalize the property name storing
the name of the base model to `settings.base`.
- Add a ModelDefinition class to encapsulate the model schema handling
- Add a Connector class as the base class for connector implementations
- Optimize attachTo and mixin
- Rename some properties/methods
Raymond Feng
Clark Wang
Christian Enevoldsen
Miroslav Bajtoš
Fabien Franzen
Ritchie Martori
Raymond Feng